would you find this insulting?

Yes it is. I did not add this earlier but my T does use Yiddish expressions a lot and uses them in context and correctly, so I do think she knows full well what the term means....and yes, a schlub is a far cry from a schmuck. A schlub is more akin to a nebbish.

Man sometimes I wonder why I was raised in a family where it seemed vital to learn Latin and Yiddish.

Yiddish was the second language on the Ed Sullivan show