Can we create Math symbols/expression in the Markdown editor?

I don't think so. I believe the current best practice would be to use mathml or mathjax. MathML wraps its content in <math> tags, but browser support seems inconsistent and math is not a permitted html tag in Forem's markdown processor.

A quick check (including the "math" tag for a^2 + b^2 = c^2 from the example on the mdn page and previewing) shows that's not automatically happening.

I think you can include math expressions as attached images, but I don't know of a way to add them directly. There might be one of the embeddable sources (via a liquid tag) that have better support for this.

MathJax seems like a good option here (it supports LaTeX math mode commands) - $$ a^2 + b^2 $$ syntax, but that's not currently enabled (It would require adding a script to the editor and article/comment view pages). I don't know that there have been many other discussions about this, but it seems like it could be a good addition.