Dell had a Linux line some years ago where everything worked out of the box, never got the popularity needed to keep it alive.
System76 has Pop!_OS so that they can provide great out of the box experience with their computers, but they are not as big as other vendors.
A good way to really get a product like that to mass market is to make it available in general stores (Walmart, Best Buy, Etc.), the problem is that most of those customers will not understand why their system is so different and they cannot install that MS Office 2003 they have always used, or that Norton Antivirus that their cousin’s son recommended to them 10 years ago that was working fine on their old computer.
And then you have the younger generations that use every other device but a computer. They’d rather do all their school and college work on their phones and tablets rather than open a laptop, if they even know how to use a computer (you’d be surprised how many don’t even know how to use a computer).
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.