Hello! I’m really sorry but are there any fics where Tony hits his head (or someone casted a spell on him) and suddenly lost his intelligence? Thank you in advance!

Other than de-aged type stuff where he isn’t really rendered less intelligent, just younger, there are only a few kind of along those lines I can think of, but here you go.

COMMENTS AND KUDOS ARE A SIGN OF INTELLIGENCE

Absentia by @kiyaar:  (no summary) includes brain injury and aphasia

C is for Cat by GlutenFreeWaffles:
The thing about permanent brain damage is that it typically doesn’t have regards for the complexities of marriage. (this is the one that I immediately thought of since it really fits your ask)

Words (series) by @missbeckywrites:
A sudden accident leaves Tony unable to speak.

(not loss of intelligence, but aphasia)

how deep is the ocean by katydidmischief:
Tony is silent the first week and there’s plenty of ill-made jokes
behind the man’s back, gleeful comments about the blessed quiet. Then
the liquor cabinet empties faster than ever before and there’s only one
person who could be consuming the bottles at such an alarming rate.

Alexia by everythingispoetry:
Tony was ten when he lost the ability to read after an accident
involving a brain injury. He’s managed to hide that information from
almost everyone for decades and he intends to keep it that way, but then
the other Avengers move in and Tony realizes that inviting them was a
very bad decision.

(not really less intelligent, just very insecure about his illiteracy)