> Seems they're perpetually short of funds, but
Not really. In 2015 the Wikimedia Foundation made 76 M$ in support and revenue, and spent 53 M$. https://upload.wikimedia.org/wikipedia/foundation/0/0b/Audit... p5
(In fact, I don't really understand why their donation campaigns are so obtrusive, given that they could afford to receive 30% less even with their current level of spending. I guess it is understandable that they want to receive as much support as they can, but I wonder whether it really serves their long-term interests, as it probably sends the wrong signal.)
> a "funded by X" tag at the bottom
Wikimedia doesn't allow ads on its properties (which I think is very important), and I would say that advertising donors on articles would be against that goal. (Think about what happens when an article gets edited in a way that isn't approved by the advertised donor.)