in my question in my ell.stackexchange account, I want that look like user$20445$ instead of user20445.
The numbers are not inappropriate. This is a standard font. You can change the font on your side, if you want to. Of course basically everybody else will still see it differently. Some instructions are here How to change all fonts on Stack Exchange sites?
You should not use MathJax here for writing these numbers either. That is inappropriate. You should use MathJax only for Mathematics.