You can tell MathJax to use the same font as is used for the surrounding text by the following command:
$\style{font-family:inherit;}{\text{Some text}}$
This gives:
$\style{font-family:inherit;}{\text{Some text}}$
This assumes that what you want is to have the same font as the text, which happens to be a sans-serif font at the moment. In case this main font should change in the future or somebody changes it locally via using a custom style-file the text above will also change.
If the intent would be to "force" a sans-serif font, just one that usually is less bold, then this can be done using:
$\style{font-family:sans-serif;}{\text{Some other text}}$
This gives:
$\style{font-family:sans-serif;}{\text{Some other text}}$
I learned this from an answer by regret on math.se, and the comments below.