I've RPed with plenty of people who had different IC names than the displayed name. And honestly, it's kinda silly if someone is getting mad about that because it's a clear case of metagaming in my book to be judged by the display name before an actual introduction has happened.
In my experience though, most people do tend to have their IC name as their display name. But that doesn't take into account nicknames or any other changes. It makes even less sense if you do introduce your character using an alias, which is your display name. It sounds...perfectly fine?
To answer the last question, while I don't think it's particularly common, an Ishgardian using an alias is likely no less rare than any other person using an alias. It's a matter of personal preference. I'd personally be interested in learning why such an alias is there.
In my experience though, most people do tend to have their IC name as their display name. But that doesn't take into account nicknames or any other changes. It makes even less sense if you do introduce your character using an alias, which is your display name. It sounds...perfectly fine?
To answer the last question, while I don't think it's particularly common, an Ishgardian using an alias is likely no less rare than any other person using an alias. It's a matter of personal preference. I'd personally be interested in learning why such an alias is there.