I wouldn't say no, it's just that there are a ton of classifications that fit him better

What kind of categories?

Trickster

Manipulator

Torturer

Dark Lord

Entity

Horror

Sadist

I love a good mastermind as much as anyone but I feel like that's an overused category.

He can also fit in Opportunist, Thrill-Seeker, Giants, Parasite, Paranormal, or some similar.

I feel like he's not even in the mastermind category, despite being really smart. Maybe it's because he doesn't have a backup plan besides killing kids every 23 years to survive.

To qualify as a mastermind, such characters have to be rarely caught off-guard and must have a plan B, C, or D to fall back on should their original plan be foiled.

I dont think IT qualifies. He relies more on his powers than his intelligence, and yes he’s definitely cunning, but that fits more under Master Manipulator - which his original version already is.

His original page even outlines that he’s immature, arrogant and shortsighted:

Also, for all Its bravado and an impressive medley of powers, It is also rather hedonistic and emotionally immature. It often does whatever appeals to It instantly and never seems to plan ahead or take precautions. It also has a childlike need to cause a commotion and get attention rather than get on with killing Its opponents. Its arrogance and shortsightedness are ultimately what had led to Its undoing. It felt the need to take on all the Losers together to reaffirm Its supposed invincibility rather than accept Its limitations and pick them off while they were separated.

This isn’t Mastermind behavior…