What people mean by an aesthetic username
It is a handle that carries a mood: cool, cute, soft, or a little wistful. The goal is a name someone can read and remember, not a string that only looks busy.
A simple way to choose one
Note two or three interests, or the feeling you want the name to suggest.
Keep it short enough to say out loud.
Check that it still makes sense if the fancy letters get stripped back to plain text.
A few sparks, not a giant list
Use these as shapes, then change them so the name is yours:
Soft: linenhour, palemonday, milkglass
Cute: peachpocket, bunmail, tinyorbit
Cool: nightmeter, velvetstatic, lowsignal
For more mixes, open the aesthetic name generator. If you also want the letters themselves to look styled, run the result through the font generator.