Bill Cosby is grumpy, not angry. Grumpy is what happens when you get old and the world changes, and suddenly no one seems to like the music you like, or the clothes you wear, or the rules of etiquette and grammar that you consider fundamental. Angry is different. Anger is passionate and engaged and political, and it can change the world. Grumpy people tend to eat themselves up on the inside, while angry people take to the streets, tear down walls and topple governments.

A new exhibition at the Smithsonian’s National Museum of African Art draws heavily on art collected by Cosby and his wife, Camille Cosby, since 1967. To celebrate the 50th anniversary of what was once called the Museum of African Art, the NMAA has invited the Cosbys to lend part of their collection to an exhibition, “Conversations,” that juxtaposes African art from the museum with African American art from the Cosbys.

This is the first time that the extensive Cosby collection, which includes more than 300 works, has been on public view. Given Bill Cosby’s epic grumpiness about some aspects of contemporary African American culture, it’s not surprising how tame most of the work in the Cosby collection is and how much of it is bathed in an idealized sense of the past. Race and racism are occasionally present as subjects, but there is a lot of nostalgia, too, and nostalgia is a powerful way that grumpy people sustain their grumpiness.

In an interview published in the exhibition catalogue, Cosby repeatedly explains that he isn’t interested in “angry” art.

“I only picked artworks that gave me a feeling of calm, because I couldn’t stand to come home to the stereotypical images of mother and child or angry black people after dealing with some of the racist people I encountered during the day,” he says.

He is referring to his early years as a collector, when he was a fast-rising star with a burgeoning television career. Later, in 1978, Cosby enlisted the professional advice of David Driskell, an artist and prominent scholar of African American art, who helped focus the collection on important historical works dating back to the beginning of the American republic. The collection now includes prized pieces by the great late-19th-century and early-20th-century artist Henry Ossawa Tanner, the portraitist Joshua Johnson (born before the American Revolution) and Robert S. Duncanson, who aligned himself with the Hudson River School.

If you compare what is on view from the Cosby collection with what was seen in a 2011 exhibition at the Corcoran “30 Americans,” which presented the work of top contemporary African American artists, about the only point of overlap is Robert Colescott, who died in 2009 at age 83. The Cosby-owned Colescott on view, “Death of a Mulatto Woman,” is one of the few pieces from their collection that stakes out a strong political view of the world. There is nothing in the collection from widely collected and respected black artists such as Glenn Ligon, Kara Walker, Carrie Mae Weems, Hank Willis Thomas, Nick Cave or Mickalene Thomas, to name but six prominent artists.

The museum explains the joint exhibition as a return to its roots, when it was an independent museum on Capitol Hill that included African and African American art in its collection. When it became a Smithsonian franchise in 1979, it focused exclusively on the arts of Africa, and much of its African American collection was given to the Smithsonian Museum of American Art. So, the current exhibition is a nod to the organization’s past and an opportunity to put the art of Africa “in dialogue” with the art of the African American diaspora.

Dialogue is an overused idea in the art world, and it is often deployed to make the juxtaposition of unrelated or only superficially related work sound more intellectually substantial than it is. Two of the strongest “dialogues” in this exhibition both involve the art of Driskell, whose work is directly engaged with African themes. His 1978 “The Green Chair” becomes more than a finely wrought and moody evocation when seen next to a high-backed stool (by a Luguru artist) and a low pedestal stool (by an Asante maker); the former is connected to initiation rites and social power, the latter “symbolizes the soul of its owner,” according to the museum’s wall text. So Driskell’s chair comes from a tradition in which chairs are not just chairs.

But not many of the other dialogues establish much more than a vague sense of formal similarity. Worse, in many cases, they underscore the weakness of the work in the Cosby collection. Elizabeth Catlett made some powerful prints in the 1960s and ’70s, directly engaged with issues that shaped the African American experience, including poverty and injustice. The Catlett sculptures included in this show are at best pleasing and decorative, certainly derivative, and without any of the raw force of her earlier work. Unfortunately, they look even weaker than they are when juxtaposed with truly evocative pieces from the Smithsonian collection.

A room specifically given over to art with a political edge is dominated by the South African artist Johannes Phokela’s “Cuts,” also from the NMAA collection. This piece — in which a grid of frame-like forms is unable to suppress the blunt power of physical wounds painted and stitched with string — makes a nearby Cosby collection abstraction by Keith Morrison seem mute to the point of meaninglessness.

Again and again, throughout the exhibition, the works that take your breath away are not works from the Cosby collection. That’s not to say that the Cosbys haven’t acquired important paintings, especially the historical pieces from Tanner and earlier artists, and a relatively robust showing of work by familiar names of African American art, including Romare Bearden, Horace Pippin and Jacob Lawrence.

There are also some nagging ethical issues to be considered. Museums such as the Smithsonian should not be exhibiting private collections unless they have been or will be accessioned, and even these kinds of shows don’t usually have much focus. The appearance of a private collection in a museum can substantially enhance its value at market, and given that, any future relation between the collector and the museum will be seen under the shadow of a possible quid pro quo. Appearances matter, and while corruption is the expected norm in politics, commerce and religion, museums should be above even a whiff of this sort of thing.

Also, the exhibition includes a painting by the Cosbys’ daughter Erika Ranee Cosby. It isn’t bad, and in some ways feels of a piece with some of the other Cosby collection works, which push gently in an expressionist direction without ever crossing any lines of decorum. But when you add the appearance of nepotism to the complicated ethics of showing a private collection in a public museum, doubts about this whole project multiply.

What was the NMAA after in all this? Perhaps it hopes to introduce new audiences to the museum’s core mission through the increased foot traffic that Bill Cosby’s celebrity may generate. If so, it’s particularly unfortunate that the show opens with renewed attention being given to allegations of rape and sexual abuse made against (and denied by) Bill Cosby over the past decade. Less than two weeks before the show opened Sunday, this newspaper published a story with the headline: “Is the world starting to turn against Bill Cosby?”

Collector shows naturally invite viewers to analyze the art for clues about the character and tastes of the people who assembled the collection. The Cosbys have a clearly expressed preference for figurative and narrative art, and they say part of their mission is to rectify the omission of important African American names from the history of American art. But they seemed to have lost interest in the ongoing project of American art about the time they started collecting it.

For a decade or more, Bill Cosby has also publicly criticized African American culture about drug use, dress, profanity, teen pregnancy and other bootstrap issues he associates with personal responsibility. He is not alone in this. President Obama has also been critical of some aspects of African American culture. But when you say controversial things, tone matters, and many Cosby critics feel his tone is smug and out of touch and insufficiently attentive to the profound structural and systemic challenges faced by many black people today, including inequities in education, health care and access to resources.

His art collection seems to confirm some of his critics’ misgivings. If nothing else, one can feel a little sad and a little troubled that he isn’t more interested in the myriad creative directions taken by African American artists since the middle of the last century. Go back to that thing he said about coming home after dealing with racists all day and not wanting to see stereotypes or images of anger. It sounds like a man scared of his own anger, scared of where it might lead him, scared it might hinder his ability to be successful and responsible in a racist world.

Anger can be all-consuming. But the history of African American art not collected by Cosby is the history of anger that is productive, incandescent, transformative and liberating. Anger is not always a dead end. Grumpiness, however, may be.

Conversations: African and African American Artworks in Dialogue from the Collections of the Smithsonian National Museum of African Art and Camille O. and William H. Cosby Jr.

is on view until Jan. 24, 2016, at the Smithsonian National Museum of African Art, 950 Independence Ave. SW. 202-633-1000. africa.si.edu