Comics who parody bad performers always run the risk of being too realistic in their imitations. The Duck's Breath Mystery Theatre repeatedly ran this risk at the Cellar Door last night, sometimes overcoming but too often succumbing. Many times during the opening show of their three-night stand, the group seemed more like a subject for parody than practitioners of the art.

Jim Turner did a solo turn as "Randy of the Redwoods," which mocked the affectations of hippie folksingers. Occasionally he scored real laughs, but often he was as painful as real-life bad singers. "Sonata My Fault" took off on Ingmar Bergman films with no insights better than stiff Swedish accents. Much better were the skits on Catholic elementary school sex education (with a feigned "spermazoid") and on the country-western singing Tranvestite Farmers Association of Argofay, Iowa.

The five men in Duck's Breath formed the company at the University of Iowa in 1975 before moving to their present base in San Francisco. Their show consists of about 40 minutes of short skits and then a longer skit of about 20 minutes. Last night the longer skit was a convention of incompetent professors discussing and performing "Hamlet" under the direction of Dr. Cliff Notee.In the pinball style of much contemporary free-association comedy, they sometimes rang bells but often went down the drain.