If you are using Linux and your MathJax fonts look ugly, and if you are certain that MathJax is correctly configured on the webpage you are accessing (e.g. by checking that things look fine on another device or browser), then your browser is probably selecting STIX fonts which are installed on your system instead of the ones offered by MathJax. The simplest way to solve this problem is by removing these STIX fonts. On Ubuntu/Debian, this can be done with the following command:

sudo apt remove fonts-stix

You can now see if this was the root cause of the problem by reloading the affected webpage with Ctrl+F5 (this will force your browser to bypass its cache).