very limited ability to self-brand

I mean, yeah. Especially when the content of your flawlessly customised site is federated to thousands other activity pub enabled sites with different stylesheets and aesthetics. That isn’t a problem with Mastodon per se, it’s just the nature of federation.

I do agree with your broader point that Mastodon has become synonymous with fediverse microblogging, which again is what most people associate with the fediverse, period.