Short answer? It's normally used against conservatives, but cliques and purity politics (both literal politics and not) do come into play on occasion.
Longer answer: Lemmy was originally founded by a bunch of Marxist-Leninists and socialists of similar stripes (that's what the .ml stands for), and early adopters often made up some form of minority group/outcast - LGBTQ and the like. This has led to a very zero tolerance policy towards conservative "talking points" and the usual bag of tricks that they employ when attempting to colonize an area/group. Especially as Reddit has further enshitified, but even before then Redditors were generally thought of more in terms of r/the_Donald subscribers rather than as disparate groups from across the political spectrum.
There are of course the "joined Lemmy before it was cool" groups who resent the growing popularity of the platform - especially after the Reddit API exodus that brought you and me here - but I think they're largely relegated to the parts of Lemmy that most of the instances defederated from. Some of those places are basically the leftist equivalent of 4chan, and would absolutely use it as an insult if you failed their political belief purity tests.
In short, basically everybody would use it for a Trumper, but a small few might use it on me if I were to say something like that I think that dbzer0's support of genAI inherently makes the instance pro-corporatism so long as they're the ones benefitting from stealing labor from workers, and an even smaller few would probably use it simply because I started using Lemmy during the Reddit API fiasco.
