Most curses look horrid. How come some curses look humanoid?

Sometimes it's because the subject from Japanese folklore was humanoid, sometimes it's a creative decision based on the nature of rhe curse. Mahito is the cursed manifestation of the human soul, and he's juat a baby curse.

Does Sukuna only look like Itadori because of the vessel thing?

If Sukana were to be in another vessel he would look like that person unless he fully manifests - then he'd look like his old 4 armed 2 faced self.

If Jujutsu High had 6 fingers already, why didn't they just give them to Itadori too?

They fear feeding him too many fingers too quickly could give Sukana too much power.

Why can't they just... Desintegrate, burn, destroy, cursed objects?

Why doesn't Lex Luther just shoot Superman with an AR15 using hollowpoint rounds? Is he stupid?