Well, since Fozzie was only plying a suporting character in MTI, they had to give him something to play off of, that still makes him funny, hence Mr. Bimbo.
I don't understand why people complain about the post Oz Fozzie so much. I don't think they were trying to make him into a half-wit, they're just trying to make him as pleasently unaware as he was in the previous Muppet franchises. Personally, I think they went a little overboard, though, especially in MFS when he washes the invisibility spray off his hands because it's something "Mama told him to do after he used the bathroom."
I mean, let's face it, Fozzie is a little goofy. He'd get on Kermit's nerves all the time on The Muppet Show. He's perpetually aloof and his need to make jokes never has an off-switch, so don't call him stupid, call him funny.