A ratings hike and a network renewal. What's with the sudden rebound of NBC's talk show Late Night With Conan O'Brien? We believe the answer may lie in chubby sidekick Andy Richter, who, in just one short year, seems to have grown on people. In fact, the mere mention of his name now elicits audience applause and screams. ''I was always confident that people would like me,'' says Richter, 27. ''I have friends who are funny and interesting and they seem to like me.'' According to an informal poll, interested observers agree that Richter has newfound appeal:
*The Fan: ''Andy is funnier than Conan. He's like Jason Alexander on Seinfeld. He's a slug and people identify with that.''
* The Media Analyst: ''I'm always proud of America's ability to clasp to its bosom the strange characters that inhabit late-night television. In the early days, David Letterman was annoying as well.''
* The TV Critic: ''He's another bad part of a terrible show. He's just one of those pieces of driftwood we find on our TV screen from time to time.''
* National Association to Advance Fat Acceptance: ''Fat people need to see people who feel good about themselves,'' says a spokeswoman. ''Large men need positive role models too.''
* Ed McMahon: ''It's easy to be critical of Andy being second banana is not an easy job. But I like Andy. When he was on my show [Star Search], I said, 'Andy, I toss the torch to you. You now carry the mantle!'''