Earlier quoted context omitted.
What makes you say it's not a good mobile protocol? It had some problems in the early 2000s on mobile, but these days it has stream management similar to TCPs and XML compresses very well. As I mentioned in another comment, there are people running it over very low bandwidth HF radio, if it works well there it works well over phones, but I'd be curious to see why people still think otherwise. (Google Talk, naturally,…
What service is using it at scale these days? I can't really think of a popular user. After a certain point it just becomes a headache. When I was working with ejabberd clusters servicing mobile clients it worked but I've had a lot less pain just using a custom protocol over websockets. My personal experience also seems to be mirrored throughout the industry as all the big players eventually move away from xmpp.
Riot Games is also using it for the messaging part of League of Legends (https://engineering.riotgames.com/news/chat-service-architec...)
XMPP is also used by military organisations such as NATO (https://www.isode.com/solutions/military-xmpp.html)
The sad truth about XMPP is that it just works, but it has a very bad publicity because there's just no marketing around it, and it's never used directly by users. So it suffers from that wrong image that is just completely outdated.