I generally disagree with all the ideas that MC is dumb. Even if he is saying dumb things at time and doesn't have a mind for academics, he is quick on the uptake and he has strokes of genius.
In all the flashbacks to his earlier years he is very sharp. The amount of self reflection is very uncharacteristic of his age and he is very savvy. He also has very good control of his emotions and knows how to survive on his own. If I had to rate him only as a kid, I'd even call him a genius. ( I also fully relate with his ability to sleep in class and get "good" grades. I was also a long haired class sleeper with good grades ^^ )
As far as post-infection intelligence is concerned:
1. Despite having 0 combat experience, he quickly adapted to the challenges without much time to prepare.
2. He is very good at applying his powers in out of combat stuff, like playing poker.
3. He managed to keep his identity hidden despite being exposed to so much publicity, as a direct result of his preparation.
4. His approach to fighting Jake was genius and he went into it with the disadvantage of not knowing the extend of his powers while Jake knew about him both from Ella and all the videos of his superhero persona.
5. His mantis punch which was catalytic against Valravn was the result of tinkering and engineering. He adapted foreign physiology to his own in a way that it's not just effective but absolutely fearsome to face.
Despite how "simple" MC may seem at times, his way of fighting is complex and he employs multiple tactics, making him a very difficult enemy to face even without the regeneration. It's technical and precise, even if he has to rely on his regeneration to overcome his lack of experience. A simple guy would just build a big fist. MC's approach was far more methodical.