Few performers have had as uneven a TV career as Martin Mull. These days he can be seen spreading his amusing sneer across ABC’s Roseanne, playing Barr’s smug, supercilious coffee-shop boss. A few weeks ago, Mull matched Chris Elliot smirk for smirk in a riotous guest shot on Fox’s Get a Life. But Mull’s TV appearances haven’t always been so enjoyable. Sure, he was perfect as the smarmy Barth Gimble on the enduring 1977-79 talk show spoof Fernwood 2-Night, now on Nick at Nite (weekdays, 11:30 p.m.-midnight). But even before that, when he made his living on concert stages singing the dry, mean-spiritedly witty novelty tunes he’d written, Mull often seemed to radiate the message I’m Too Smart for My Own Good. His trademark irony curdled when he starred in fetid little sitcoms like Domestic Life (1984) and His & Hers (1990), and he’s often unbearable playing himself on talk shows like Late Night With David Letterman. You get the depressing feeling that Mull’s crinkly-eyed contempt for his audience isn’t a spoof of star condescension — it may be the real thing. As Roseanne’s chief nemesis though, Mull is putting his artistic hostility to good use.