Pokécharms

Cuz it looks like one and it never got mentioned as a Legendary, Ultra Beast or anything else for that matter...

Well without the things I've mentioned up then it can either be a Legendary or a Mythical.....

It's mostly the appearance and the part of the game it actually appears in. The game itself notes its resemblances to the Ultra Beasts, though it also states it isn't one itself - although that's where you get into what defines Ultra Beasts.

Loosely defined they're Pokemon native to another dimension - a specific one or a set of ones, we're not sure - that came into the Pokemon world through an Ultra Wormhole. Note that lore-ways Cosmog etc. fall into this bracket and yet they aren't defined as Ultra Beasts. It's quite possible that Necrozma is a similar deal - it's implied that it's been around for a long, long time.

The 'Necro' in its name suggests to me that it's meant to represent a long-dead or dying star. Makes me wonder if that's what happens to Cosmog/Cosmoem that aren't exposed to Sunne/Moone energies. Either way I guess they'll clarify more about the thing in the next game. At least, I hope so.

I checked the dex entry and it says something along the lines of being made of the remnants of an Ultra beast, so it used to be an ultra beast now it isn't.... but doesn't answer the question of What is it right now..... We have nothing that defines a legendary from a mythical....

"Makes me wonder if that's what happens to Cosmog/Cosmoem that aren't exposed to Sunne/Moone energies." Huh, now there's an interesting thought. That twist could make for an incredibly heartbreaking plot in Stars...