Eh, if an instance allows trolls, then that instance typically gets defederated from very quickly (at least it does on my instance). The only reason it is an issue now is that two of the big three instances (.world and .ml) have very lax moderation standards. If the lemmy-verse grows to the size of Reddit, then two lax instances won’t be as big of an issue.
Activitypub accounts are portable actually. Lemmy is not as slick as Mastodon with this feature yet but will likely get there.