David Tua was pretty much a dullard in the ring. When plan A failed, he had no clue as to what to do next. (See Lennox and Rahman 2 fights for examples.)
I assume we are talking ring smarts here? It has to be Frank Bruno. Frank had skills and a good punch, but was completely clueless when the other guy hit back, or god forbid, hit him.
What about Iran Barkley. One of my favorite fighters sure, but he didn't seem to be all that smart in the ring, preferring to use brawn instead of brains.
That's because there's an often made confusion between Emotional Intelligence (EQ) and Intelligence Quote (IQ). On an EQ test, Ali his prime would get the Einstein score; he knows like no other how to play the public, he's charismatic, funny and his self confidence is off the chart. Who else would've not only dared to do that against Liston without skipping one beat, but also succeeding in it by making Sonny intimidated? When someone comes across as charismatic, funny, handsome, witty and quick in remarks, he's nearly always judged as being very intelligent (especially by women), while none of the traits shown are directly related to intelligence. Ali probably couldn't solve a simple math problem to save his life. But that's a different area.
Bless him, but I always thought Ray Mercer was guilty of being pretty dumb in the ring in a few fights. He would rely on his durability getting him out of jail a little too much when he should've been scoring early points.
Bruno just had zero survival instinct and a fear of being hit because of being abused as a kid. He came off as fairly intelligent in a special I saw of him.
Dumbest in the ring or dumbest out of it? Perhaps the fighter who played the weakest hand given their ability, both in career terms and financialy was George Godfrey. Every heavyweight on the planet feared and avoided the man. Some oponents said that he was better than Jack Dempsey. The man was inumerate so he never saw most of his fight purses and threw fights against guys who might have directed him to title shots. One of the sadest stories in boxing.
The is inteligent and that is part of his problem. Children born in the gheto with a genius IQ never have any stimulus for their inteligence and so they channel thier it into self destructive paths and generaly become persistent ofenders. If you could IQ test Tyson I personaly guarantee that he would score high.