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.