In general, I agree. Diversity is a good thing, and I tend to prefer giving users tools for self-moderation rather than seeing instance admins use a blunt instrument like defederation which will inevitably have spillover consequences for communities that aren't even associated with TheDonald.
HOWEVER, this is still a good debate to have. The issue shouldn't be "allow all federation" vs "allow no federation" at the instance level, but rather "where do we draw the line?" Too lax and you've got child porn, hate speech, and weaponized disinformation. Too strict and you stifle free speech. I think we can all agree that a middle ground is the goal. So where is the line in this case?
Political communities? Great! Bring 'em on. We need an open "marketplace of ideas." But you can have a political debate without devolving into hate speech, provably false disinformation, calls to lynch politicians, and doxing. It doesn't matter if they're coming from the left or the right, death threats are not okay. That's not a political statement. It's just a fact. TheDonald has a long and sordid history on Reddit, and I doubt very much that it's even capable of staying on the right side of that line. But we should judge their Lemmy community by their moderation policy, not their history on another platform. Do they regularly allow hate speech? Death threats? Provably false disinformation? If so, that's exactly why defederation exists. Yes, there might be spillover consequences for the other communities on that instance, but if the instance mods don't have a problem with hate speech in their communities then it is an instance-wide problem that should be addressed at the instance level.
So far they're tiny and haven't done anything one way or the other. I don't think anyone should kick them out just for existing. But I do think we should talk about our community-wide lines in the sand, and then hold instances we federate with to those standards. Including TheDonald.