In the past there were bots spreading ads for pills and stuff, but now most instances ask to tell a bit about yourself before registering and don’t federate with instances that don’t. That eliminated practically all of it as far as I’m aware. However sometimes there are accounts like these:

that go around spamming content that intentionally seems to aim to disturb users. Not sure if they are bots or humans, but they are banned relatively quickly.

Regarding the karma-farming bot accounts commonplace on Reddit, I think there’s no incentive for the creation of such bots, since most Lemmy instances do not have a total user point/karma count like on Reddit.