Bette Midler, whose CBS sitcom "Bette" got off to a strong start but has been slipping in the ratings ever since, is not only losing viewers (it's currently No. 71 out of more than 100 prime-time shows ranked by Nielsen) but also one of the key cast members on the show. Actor Kevin Dunn, who plays Midler's husband Roy on the show, is leaving after 10 episodes. "It was a mutual decision between him and the producers," a publicist for the program told reporters yesterday. No reason was given. (Dunn's departure is the second cast exodus on the show, which replaced Lindsay Lohan with Marina Malota after only two episodes.) Meanwhile, Bette was doing some talking of her own yesterday. On the ABC daytime chat show "The View," host Barbara Walters reminded Midler that she recently said that doing her sitcom would be "a piece of cake." Responded Bette: "No, it's not very easy. It's . . . I started to say it's a lot of fun, but I really can't say that, either. It's really much, more stressful than I thought it would be." Earlier this week on David Letterman's show, Bette talked about her own program and said, "I'm like a dung beetle pushing this ball of dung up a mountain." She then called the sitcom "the lowest thing that ever happened to me in my life."