Auteme
I certainly don't mind 'em, though I am wary of people who do it just to make their character seem cool and legendary. Generally speaking, if you are a new writer who nobody knows, it can look tacky if you go around claiming to be "the Savior of Coruscant" or whatever. If your character has done something worthwhile in an RP, sure. The main thing is you have to be able to back up your claim to whatever title.
On the other hand, you can have titles not based on something your character did, but on specific traits they have. I've occasionally referred to Starlin as the "Silver Dragon" which is a reference to his having been a member of the Silver Jedi, and his ability to breathe fire/smoke. I think somebody once referred to him as "Jokerman" at one point, because of his constant quipping.
It also kinda overlaps with euphemisms. And now this is reminding me that I need to eventually give
Kai Bamarri
a real name. "Kai" was already taken.
So yeah. As with most things, it's fine as long as you don't abuse it or make it cringe.