I have personally not checked this template but one of friend suggested earlier for me. But our project get delayed so have not checked it. But you can check template - github.com/zacharystenger/mern-starter-JWT-auth

I haven't found a specific boilerplate for MERN yet (more of a SQL guy myself), but Meteor is an all-in-one framework that has things like authentication baked in.

Hashnode has their own mern-starter and the README references there being a Material UI and JWT auth variant, I can't find them on a quick glance, but that might be another good shot.