yEd leverages the HTML support of the Java framework which in turn is based on HTML 3.2 and does not support <math> tags, unfortunately.
I am afraid there is currently no good way to display complex mathematical formulae in yEd.
While supporting LaTeX would be great, the libraries I know of either use licenses that are pretty much incompatible with closed-source or do not work for on-the-fly rendering. Implementing LaTeX support from scratch is very much out-of-scope for yEd.
