You're confusing UX with UI. UX = user experience, the entire experience, UI = the interface.
UX is the entire user experience, and for example for joining reddit, you go to reddit.com and join.
For lemmy you learn there are dozens of large instances, with intricate politics between them and if you join the wrong one everyone thinks you're a tankie....
That's terrible and i can imagine people are put off by it.
The interface of lemmy itself is indeed ok, and is close to old reddit, which at least the people here prefer.