Absolutely it’s a populars ads network for mobile games and can be used without the unity engine

Is it common though? I’m wondering if maybe they just consider it to be a “related product” because 99.9% of the customers are Unity engine users.

I don’t know any actual numbers

Yes any mobile games (be it made by unity or other engine) that display ads most probably have unity ads in their waterfall it is very popular

Duolingo uses it for their in app ads.