Gov. Harry Hughes's choice to head the massive Department of Human Resources, Ruth W. Massinga, used the public announcement of her appointment today to come out strongly against Reagan administration cutbacks in human services areas.

"I think that the president's policies are misguided and have not been demonstrated to work," said Massinga, 42, who next week will give up her post as head of the social services section of Human Resources to join Hughes's cabinet. "Every informed person out there knows this although I wonder if President Reagan is one of them."

Massinga's comments echoed criticisms by her Democratic boss, Hughes, and the man Massinga is replacing as secretary of the department, Kalman R. (Buzzy) Hettleman.

Hettleman told Hughes several months ago that he would be resigning from the department, which supervises 7,000 employes and has an annual budget of $500 million, to teach and take a break from government service. Hettleman, whom many considered the strongest member of Hughes' first-term cabinet, had backed Massinga as his successor.

Some human services advocates questioned today whether Massinga, who has never managed a bureaucracy as cumbersome or hard-pressed as Human Resources, would be as effective or creative as Hettleman in dealing with expected continued cuts in social welfare programs.

"She's done a good job in the past; now the question is how will she do as leader of the pack," said Del. Charles J. Ryan (D-Prince George's), vice chairman of the appropriations subcommittee on education and and human resources.

Several advocacy organizations that worked with Massinga during the last three years when she headed the department's social services administration, reacted positively to the news of the appointment. "She's intelligent, she's articulate, she's an advocate for her constituents: parents, children and poor people," said Sandra Skolnik, executive director of the Maryland Committee for Children.