Isekai Tensei Slum Machi kara no Nariagari - Ch. 10

[AlvoradaGD]

Why does the MC feel smart and dumb at the same time?

I'd say it's as combination of the conflict of having memories but also being reborn in a young body (which does get affected by hormones, emotional bursts and all that which grown-ups learn to cope better) and also the way how most isekai choose to portrait "otherworldly knowledge" - which is being able to recreate guns, modern weapons, dishes and all kinds of stuff easily. Compared to that, a MC stripped of resources and slowly improving their situation instead of skyrocketing seems dumber.

edit: Their airhead expression might also add to the confusion.