
Using an alias could make sense for a lot of characters. There's nothing wrong with it. I'm not sure why anyone would really take offense to you naming your avatar based on your character's alias rather than her birth name, that's just kinda crazy, and craziness on that level isn't really an indicator that there may be many out there who feel the same as that person did. That being said, when I have a character who uses an alias, I still usually name the avatar their actual name. I don't know why; it just feels more accurate and authentic.
I can't blame anyone for shying away from that, though. Having your character's real name in your nameplate can be fuel for others to metagame, purposely or accidentally. I.E. My character Aelius Corinthius is a Garlean man who was undercover in Eorzea, using the name "Ael." Someone I was RPing with, noticing his nameplate, asked me OOC if he was a Garlean, and I said yes. From that moment, her character who was very trusting and clingy toward him suddenly became suspicious for no reason and questioned just about everything he said/did, no matter how mundane.
It wasn't uncommon that people would slip up and call my character "Aelius" IC, even though they did not actually know his name IC. This girl was around in one such instance of this happening, and after "learning" his name, her character immediately drew the connection he was Garlean (I understand in a way, but I also have seen plenty of characters who aren't Garleans with Latin names, even if that doesn't actually make sense canonically). Even after I--and the person who made the slip-up--explained that it was an OOC accident and the name "Aelius" would not have actually been said IC, she still went with it and refused to stop or retcon. My character was meant to be the antagonist of a big plotline, and even though he'd done nothing to out himself or cast major suspicion on himself, I almost had an entire plot-line that was a year in the making wrecked because someone was using metaknowledge and almost prematurely revealed to everyone else IC that my character was a Garlean.
That being said... I can't blame anyone for using their character's alias for their in-game name. It's something I may consider doing in the future to avoid any more issues like this. Your nameplate being different from your character's alias can at best lead others to accidentally call your character by their birth name IC, and at worst lead other characters to immediately become suspicious of your character (even though there's no way for them to know IC that it's only an alias). So, it just sounds kinda like this person was mad they weren't given the chance to metagame.
I can't blame anyone for shying away from that, though. Having your character's real name in your nameplate can be fuel for others to metagame, purposely or accidentally. I.E. My character Aelius Corinthius is a Garlean man who was undercover in Eorzea, using the name "Ael." Someone I was RPing with, noticing his nameplate, asked me OOC if he was a Garlean, and I said yes. From that moment, her character who was very trusting and clingy toward him suddenly became suspicious for no reason and questioned just about everything he said/did, no matter how mundane.
It wasn't uncommon that people would slip up and call my character "Aelius" IC, even though they did not actually know his name IC. This girl was around in one such instance of this happening, and after "learning" his name, her character immediately drew the connection he was Garlean (I understand in a way, but I also have seen plenty of characters who aren't Garleans with Latin names, even if that doesn't actually make sense canonically). Even after I--and the person who made the slip-up--explained that it was an OOC accident and the name "Aelius" would not have actually been said IC, she still went with it and refused to stop or retcon. My character was meant to be the antagonist of a big plotline, and even though he'd done nothing to out himself or cast major suspicion on himself, I almost had an entire plot-line that was a year in the making wrecked because someone was using metaknowledge and almost prematurely revealed to everyone else IC that my character was a Garlean.
That being said... I can't blame anyone for using their character's alias for their in-game name. It's something I may consider doing in the future to avoid any more issues like this. Your nameplate being different from your character's alias can at best lead others to accidentally call your character by their birth name IC, and at worst lead other characters to immediately become suspicious of your character (even though there's no way for them to know IC that it's only an alias). So, it just sounds kinda like this person was mad they weren't given the chance to metagame.