The little mouse icon is defined by the instance frontend that's showing the comment, as a placeholder for users with "avatar: null". Functionally, it's intended for each instance/app to define how they want to render "avatar: null" user avatars (some apps, like Liftoff, just leave it blank, no mouse).

if you made your account on Beehaw and you didn’t set a profile picture, you get a cute little bee

I understand that would require setting a profile picture for people who didn't set one themselves, which, given the federated nature of Lemmy, would kind of go against the idea of each instance/app deciding what image they show for people who haven't set an avatar picture.

Ideally, I think there could be an "avatar" and a "default_avatar" option for the target instance/app to decide whether they wanted to display their own (to preserve interface consistency), or the one suggested by the user's home instance, but that isn't part of Lemmy (yet).