He's not stupid. He doesn't act stupid. It's just the company he keeps.
Jenny said (of Gan): Oh, he does act stupid. There's a lot of Gan's actions which are put down by a lot of people (including the crew) to his being stupid-- not checking to see if Jenna was behind him on Cephlon, or tearing out the computer banks in "Breakdown." But if he's doing these things, and he's not stupid, why is he doing them?
Breakdown-- he has very strange things going on in his brain because his limiter is malfunctioning. Maybe the pretty sparkling lights irritated him. Maybe he was getting revenge on the computers for all the times he didn't know what button to press. I cannot see that as typical behavior for him. On Cephlon, I believe everyone was on board ship before anyone noticed Jenna missing? Does that mean Vila and Avon are also stupid? Or that when one is on a very hostile planet one doesn't always look around to see if everyone else is okay, especially if the other person is believed to be comptetent in a dangerous situation.