How feminist is a site like Onlyfans really?

I'd say it falls under whatever value of "feminism" is the one that allows them to claim empowerment by repeatedly selling pictures of their bodies to horny simps, while playing mental gymnastics so they can act like it isn't objectification or degradation of the woman in question.

Sure, a couple of popular ones make bank but is it really such a cushy "job" for the vast majority?

You have to remember, you're dealing with the long tail once you get into "hundreds of thousands". All it takes is a couple hundred subscribers at $10/month to reach the point where the monthly payout is closing in on being equivalent to what one would make at a minimum wage job. Throw in bonus "personal" pictures/videos they can charge extra for, and any "gifts" they may get from an Amazon (or Adam & Eve) wishlist and they could rack up a decent amount of extra income for very little actual "work".