Depends on the team and line-up. (And bear in mind that most characters will be cleverer on their own, than on a team!)
Fantastic Four - Reed's a genius. Sue, Johnny and Ben, not so much. They aren't mentally challenged or anything, but they are pretty much regularly folk. (Although Johnny can occasionally be the poster boy for poor impulse control and / or arrested emotional development...)
Avengers - the Founding Five included Hank Pym, Tony Stark and Bruce Banner, all geniuses. And Janet van Dyne and Thor, not dummies, but not geniuses, either. (And Thor can range from a tactical whiz with thousands of years of experience and familiarity with advanced Asgardian tech to dumb as a box of rocks and leading with his face, depending on the writer, much like Hercules, who ranges from clever to a buffoon, sometimes in the same comic...) Later additions, Captain America, Hawkeye, Scarlet Witch and Quicksilver had no geniuses, even if Cap has a lot of experience and is a good strategic thinker and leader, and Quicksilver can process information fast.
X-Men - of the original five, only Hank McCoy is a bona fide genius. (Although mutant telepaths seem to have eidetic memories and the ability to master any skill by breathing the same air as someone who has it, making them sort of geniuses by accident because their 'telepathy' includes fifteen other random powers, other than just read and send thoughts.) Scott, Jean, Bobby and Warren are not generally regarded as geniuses, even if Cyclops has the same sort of strategic reputation that Captain America does (and Jean cheats). For the All-New crowd of Banshee, Thunderbird, Storm, Colossus, Nightcrawler, SniktBub and, again, Cyclops, nary a genius in sight. Again, none of them seem particularly thick or anything, and some have some pretty clever feats to their names (and they've all been telepathically force-fed a dozen languages and some amazing alien-tech-repair skills), but they aren't rocket scientists. (Except when they are, but that's the telepathic cram courses in piloting Shiar spaceships the Professor crapped into their brains while they were sleeping, all not-at-all-creepy-like.)
The most famous solo hero, Spider-Man, happens to also be a extremely smart. I'm not sure how much of a 'genius' he is considered to be, but he has developed at least one patentable marvelous technology (his web-fluid), so, while he's no Tony Stark, he's more *technically intelligent* (and educated) than, say, Cyclops or Captain America.
Where Marvel is just catching up is in genius ladies. Of the original top tier Marvel heroes, a fair number of the men are geniuses, but Wasp, the Invisible Woman, Natasha Romanov, Jean Grey, Storm, Carol Danvers, Crystal, She-Hulk, Scarlet Witch, etc. are generally not playing in that league. Recently, there've been a bumper crop of super-intelligent ladies (Valeria Richards, Monica Rappacini, Toni Ho, Riri Williams, Moon Girl, Nadia Pym, etc.) which feels a little bit like Christmas in July, with the fans being flash-mobbed with lady geniuses, since after fifty years of little to nothing, super-intelligent girls and women are suddenly coming out of the woodwork.