In the rock 'n' roll business, you are either on the bus or under it. Playing "Feelings" with Eddie and the Condos in a buffet bar in Butte is under the bus. Peter Frampton is under the bus. God willing, so are the Bee Gees.

But Cyndi Lauper is most decidedly on the bus, a state of the art touring model to be precise, with color televisions fore and aft and the requisite mondo sound system for the occasional boyo bacchanal on the byways.

Somewhere between the Vince Lombardi and Joyce Kilmer rest stations, Cyndi takes a long look out the window. She looks glum. The exits and industrial ephemera of the New Jersey turnpike are whizzing by as the "Fun Tour '84" makes its way. Tonight, Philadelphia. Friday, Merriweather Post.

Is it possible? Is Cyndi "Girls Just Wanna Have Fun" Lauper having no fun at all?

Just an hour before, when a guy in a union trucker cap approached her for an autograph on East 58th Street, Cyndi was her public self, a cross between Gracie Allen and Judy Holliday.

"Hey, Cyndi! Ya know I love ya," the man said. "But can ya talk a little bettuh next time yer on TV?"

"Shaw," said Cyndi with a wide, Pepsodent grin. Her speaking voice is a little bit Canarsie, a little bit Astoria. Not a bit Oyster Bay. When she's on the Carson or Letterman shows, Cyndi Lauper is a hilarious parody and celebration of every ditsy figure in movies.

Lauper is a fabulous, versatile, one-woman ongoing gig. She's writing a cookbook that includes a recipe for Pizza in a Cup and Squirrel Spaghetti Sauce. She manages a professional wrestler named Wendy. She used to wait on tables in a Japanese piano bar where she thrilled the customers with her rendition of "Wa Sare Hai Wa I'll Never Forget You ." She believes in the PEG principle of "Politeness, Etiquette and Grooming." You almost forget Lauper has a terrific four-octave singing voice.

But now she is mightily serious, even wearing socks the color of lime popsicles, and pants that look like something the Starship Enterprise is always firing at. Her hair is brilliant shades of orange, none of them of the Clairol variety. Oddly enough, she will not allow photographs. Not without her makeup.

What we are getting now is a very serious woman.

As in, Girls Just Wanna Relax a Little Bit and Talk About Other Stuff.

"The me you see on the Johnny Carson show and stuff, that's not really pretend. Ignorance can be bliss, and I just show that. It's like ripping your shirt open and saying, 'Here I am.' Everyone has the potential for innocence and everyone's got to deal with the pressures of . . . uh . . . thinking.

"Those actresses, like Marilyn Monroe or Gracie Allen, they weren't dumb. They were comics. I don't think I'm a blank. I'm not stupid. Some men figure that if you act an innocent role, you're dumb. Now Dagwood, he was stupid."

Lauper is 31, according to an old band biography. She struggled for years to make it in the business with a band called Blue Angel. She had the almost obligatory critically-successful-but-commercially-unpopular album. But her current album, "She's So Unusual," has spawned two huge hits, "Time After Time" and "Girls Just Wanna Have Fun." "She Bop" is also making its way to the top.

Lauper used to do a tribute to Janis Joplin as part of her act. And Joplin remains important to Lauper, perhaps more as a personal example than as a musical influence.

"Janis thought she was ugly and she had to endure all that abuse," says Lauper. "She came along before the women's movement did enough to protect women from abuse like that. She thought she was ugly. We're all supposed to look like this android, you know, the blond beach people you see on TV with nice, skinny bodies and shiny hair. Janis looked different and had different feelings. And she took drugs to ease the pain and it killed her. I guess if I had come along in the '60s I would have self-destructed just like Janis."

Lauper grew up in various parts of New York and those years were not unlike Joplin's unhappy times in Port Arthur, Tex.

"I hated neat hair. I hated the Little Miss Perfect look. I was the one who sat all alone at the party, got drunk and giggled and sang to herself in the corner. I was so totally alienated from everybody else that I felt like I was always out of step and I would never catch up. The doors were always closing on me. I got harassed all the time because I was so different."

It's not hard to imagine high school kids mocking Lauper for her appearance. You look at her earrings today -- they're like little Indy 500 trophies. But these days no one mocks Lauper for her clothes; they ask her where she buys them.

"It's really nice to be successful now, but when I was 17 years old I was broke," she says. "I was on my own. I was only making $95 a week doing a Gal Friday job. I ate yogurt all the time. One time they had to take me to the hospital for dehydration and malnutrition.

"I worked up in Vermont cleaning kennels and I worked out at Belmont Race Track as a hot-walker. But when a horse threw me straight up in the air, that's when I decided I was a little too small for that job. I was one of those people that was always running away. The job at the Japanese bar, though, was kind of nice. By that time I was doing things like singing as an opening act for someone like Human League at the Palladium and the next night I would be back to waiting tables at the bar. I learned a lot."

"She's So Unusual" is a huge hit, perhaps not on the "Thriller" scale, but what is? Lauper faces the testy problem of following through, of producing a second album that is every bit as manic and musical as the first. The record bins are littered with the vinyl detritus of acts that could not follow success with success.

"I think the tour we're doing now is really establishing Cyndi's credibility as a musician," says drummer Sandy Gennaro.

"The stage act is kind of like a put-on of all those beauties like Stevie Nicks or Pat Benatar," says bass player John (Just Use the Initial) K, "and the music is going to speak for itself."

As in, Girls Just Wanna Have Fun And Sing Songs As Long As Possible.

"I don't care if we call the second album 'Sophomore Jinx,' " says Cyndi Lauper. "It doesn't matter if I'm famous forever. I know who I am. I'm a singer. That's what I do. I sing.