I wouldn't wanna donate to specific fighters, cause it would be too much of a hassle to pick my favorites.
But if there was a website with a "donate to all fighters on this card" button, that would create a pool of money and split it fairly between Patreons of all fighters on the card, then I might use it once in a while.
But I would probably never be able to trust that something like that was legit and not skimming money off the top.
One question is would those people, even though they aren't paying $60 for the PPV, be willing to tip a few bucks to the fighters for the fights?
I've always found this topic very interesting.
During the heydays of PirateBay I remember that a lot of pirates were saying that they wouldn't mind paying minor artists directly, but they didn't wanna fund rich artists and greedy record labels.
One of the guys from PirateBay actually launched a donation based thing called Flattr in 2010. (Monthly donations which were basically split between all businesses which used Flattr) When it didn't really take off I assumed that donations would never really become a thing, and people didn't really care about the creators - they mainly wanted free content with as little hassle as possible.
But then stuff like crowdfunding, Patreon and Humble Bundle came along I figured it seems to have turned around.
Seems to me though that the willingness and practicality of donations are highly dependant on which type of business we're talking.
(I assume Patreon has taken off cause it's used by content creators like youtubers who create hours of content on a monthly basis - which make fans feel like they get a lot of value for money and a tight relationship with the product.
Might be harder for fighters who just appear in the ring few minutes per year)