In regards to whether the site allows microtransactions - that's not a question I know the answer to, sorry.
But I do have some concerns about using them. On Google Play or App Store, I don't mind them because I know that's usually the format.
But for Itch, I'm a bit more leisurely about it (I often browse from my laptop), and being charged for something cosmetic (like a change of character outfit) isn't something I generally look for. If the game is professional... I'd rather, for example, pay $1-$2+ for the game itself, rather than for an outfit.
It's also possible that I might not be the proper target audience for such a price model, however.
Anyway, I did see your game video, and it looks pretty cool! Nice work.