There are people whom you dislike on TV, and there are people who can't act. I think a few of you are mixing the two up. You may hate Wilmer Valderrama ("Fez" on THAT 70s SHOW), and God knows I'm not too fond of that smug little prick myself; but he _can_ act. So could most of the kids on THAT 70s SHOW (with the exception of Laura Prepon), although their TV characters' personae were often unlikable, and some of them are similarly unlikable offscreen (like Topher Grace).
People on TV who genuinely couldn't act:
*Candice Bergen. She actually can be funny, but she can't act. Those are two different skills.
*Roseanne on the first season and a half of ROSEANNE. She was absolutely godawful: she cheated too much to the camera, and smiled during her punchlines and John Goodman's. To her credit, she learned on the job FAST and was terrific by the year she won the Emmy.
*The Olsen twins, and also Stephanie Sweet on FULL HOUSE. To the Olsen twins' credit, they were under five when they were asked first to deliver lines on TV; but they were no better as they got older and did that show with Chris Siebert (and also, Raven-Symone was an accomplished child actress when she was under 5 on THE COSBY SHOW).
*Melissa Gilbert on LITTLE HOUSE ON THE PRAIRIE during its first five or six years. Both Allison Arngrim and Melissa Sue Anderson were, in comparison, accomplished child actors from the beginning, and Matthew Laborteaux was genuinely terrific from the outset of his stint on the show; Melissa Gilbert only got better the year they brought in Manly and Eliza Jane. Her brother Jonathan Gilbert (as Willie) was always unable to act.
*Jerry Seinfeld. The other three actors and the fabulous guests stars could actually act, but he couldn't. Like Candice Bergen, he can be funny though.
*Sarah Michelle Gellar. She couldn't act during her BUFFY years, and, to compensate, she picked up this. Annoying habit. Of putting pauses. In unexpected. Places in her speeches that. She learned from Alyson. Hannigan who could. Make it work. Even if. SMG couldn't.
*David Duchovny. He just can't act.