If you just want to join an instance, it doesn’t really matter if they are running Mastodon, Pleroma (or one of the forks), you will be able to follow and interact with everyone else on the microblog portion of the Fediverse. In fact from a Kbin instance you can do Lemmy communities and Mastodon microblogging from the same platform (Kbin calls communities Magazines and in the magazines are Threads, and they call the mastodon-like portion is just called microblog).
If you want to self-host your own instance, then you need to pay attention to the difference in the platfoms, Pleroma is lighter, Mastodon is more modular, and there are many forks of both each with their own strenghts and weakenesses.
If you don't like the frontend, you can use Elk, or Soapbox, or some others out there, as well as all the apps and PWAs for either platform, most are compatible with both.