Donations are already happening in the Fediverse. Lemmy,world is funded by donations to the Mastodon.world instance. Many Mastodon users donate to their instance. I give $3 per month to my instance (sfba.social). They put out a quarterly report breaking down how much cam in and went out.
I donate in order to have an ad-free experience. If the admins separate the finances of Lemmy.world and Mastodon.world I will donate here as well.