I'd agree that Sommers is very smart, up to a point. If you watch the first couple of seasons of 3's Company, she is NOT a ditz. It was her decision to make her character stupider, and the ratings took off.
But she and her doofus husband overestimated her appeal by the 5th season and asked for so much $ that it would have made the show too expensive to produce.
Thus she was written out. And John Ritter and Joyce DeWitt hated her for years, because she was messing with a top 10 show in its prime.
She DID eventually make it back to the big-time with her business ventures, but not as a big time TV or movie star.
BTW, Janet all the way, before she fouled up her face with that stupid nose job. Can't beat that dancer's body.
Cindy was good looking, but so stupid I couldn't stand her.
Terri was great looking, and smart. But by the time she came on the scene, 3's Company was running out of steam.
I never once watched that show, although I do remember it. It must have been on in my college years.