XMPP isn't a chat app, it's a protocol. If you want a chat app then you need to list clients. Which client would you use?
And xxxx/XMPP isn't on the list because when I last looked (which was some time ago) everything was unusable for muggles, same with Matrix. Responses to this poll have convinced me that the landscape has changed, although most of the suggestions are still inappropriate for someone who just want to use something, without having to understand everything about the underlying protocols.
So, back to XMPP, or Matrix. Which client would you use/recommend?
Well, for Android at least, the one I mentioned, Conversations ?
BTW, I use it with my mom. And even though I was the one that set up her XMPP account at https://jabber.lqdn.fr/my/create.php , as you can see it's quite straightforward to do ! (And other servers might have an even easier, more mobile friendly version by now, IIRC Conversations itself is suggesting one ?)
In fact I've dumped WhatsApp in favor of Conversations several years ago, when Facebook bought WhatsApp.
(I'm pretty sure that Facebook already helped themselves with all of WhatsApp's data back then, and this recent development is only ass-covering...)