MERN or Java with React? Which is good ?

When you learn React (the R in MERN) - you'll get to learn the rest of the MERN stack anyway in order to be able to work with React.

So the options are actually just MERN, or Java + MERN. And there obviously, Java + MERN > only MERN.

Because by learning Java also apart from MERN, you're broadening your skill-set, which will get you more relevant job opportunities (backend, front-end, or full stack developer roles), than if you have a narrower skill-set of only MERN (frontend developer role only).

You can choose whatever you find easier if you’re just beginning, the overall concepts for backend and frontend are the same. Changing the stack later becomes easier as your knowledge grows.