You have not given enough information about your setup. For starters, are you using MathJax version 2 or version 3? Which output renderer are you using? What is your MathJax configuration?
Have you tried the configuration setting that uses the surrounding font for text elements? How you access that depends on the version of MathJax you are using, which is why that is important. For version 2, see
(or the equivalent page for SVG output). For version 3, see
for the mtextFont option to set the font (or font list) to use for test elements, and mtextInheritFont for how to inherit the surrounding font for text element.
One of these should do the trick for you, I would think.
Davide