I liked Fawkes he wasn't dumber then a box of rocks either so I guess hes the only Super Mutant worth a damn in the Capital Wasteland.
Well you can consider Fawkes and Uncle Lou pretty intelligent as they can speak properly and at least have some intelligence.
I found Fawkes to be about as dumb as a brick, personally. He always ended up getting me killed because it seems all East Coast mutants run on:
See enemy --> attack enemy
God I hope Marcus is smart enough to not attack things crazily more powerful than he is on sight. Fawkes did that to me three times, went after Liberty Prime for no reason and got himself pwned.
The average super mutant will still be dumb, but with a new voice, obviously. So far, it looks like Neil and (of course) Marcus are the only confirmed super mutants that aren't schizophrenic or stupid. I hope for a couple more though. I expect to see plenty in Jacobstown, but not as many in State of Ultolbithia.
Tabitha probably won't be terribly stupid. Insane, sure. Schizo? You bet. But I doubt it'd be that stupid if it could organize the mutants on that mountain.
Also, there's a clear line in the Mariposa mutants. Gen 1 = generally smarter than the landscape. Gen 2 = dumber than the landscape.