XMPP is a protocol; it doesn't have a user experience. Sure, it may have been difficult to adapt for mobile, but that doesn't mean it inherently has a poor experience.
It's actually pretty easy to adapt to mobile, it may have been hard in 1998 when it was released, but these days with stream management (session resumption and TLS like packet counting) and message archive management (history and catchup) it's pretty great on mobile. The round trips to log in have also been significantly reduced (you'll incur a few more if you use SCRAM based auth, but there's always a trade off for security), though there are still a few of them for the most basic login. If you ignore TLS (which can come first so you don't have to do STARTTLS anymore) and do PLAIN auth (over TLS so it's fine) I think you have 3 round trips to login. That can be improved in the future with pipelining which has some experimental specs out.
I think it's a valid distinction. You could make the case that a bad protocol makes implementing a solid UX harder, but you wouldn't say the protocol has "poor UX".
