I asked for these recs and now I'm giving them :')
Books where MC pretends to be dense are not hard to find.
But ones where they actually are dense but still super smart are kinda hard to find, I'm putting this in order of most similar to least:
Mistakenly saving the villain (this is exactly what you asked for, genius who is dense. Great plot too)
Why are the Protagonist Gong and Shou Fighting Because of Me? (MC has a massive misunderstanding which is why he is dense, other than that, he's pretty smart)
Salted fish zombie (super smart MC, wouldn't call him dense, just hella lazy.... And well his common sense is not other people's common sense, so it gives the same vibe)
Heavens official blessing (is one of these. MC is pretty smart and has been though a lot, but he's quite dense in romance)
Little mushroom (MC isn't that smart, but he's a quick learner, and his common sense is different from everyone else. After all, he's a mushroom!)
Also note that it's very rare to have a dense MC genius who is also forward about wanting sperm....
I LOVE the trope when a genius is also an idiot and acts dumb