Which is better Mern or python?

Django and NodeJS both are used to write backend. Coming to your question, if you want to learn NodeJS then go for it but there is no rule as such that state if you know Django then you should learn NodeJS. Its your choice which technology you want to learn.

It really comes down to perfence. If you like Python, go with Django. If you like Javascript, go with Nodejs . Personally, I recommend learning Django.

MEAN Stack or MERN Stack is preferable when your web application is entirely web based and involves minimal data analysis, the npm also has a good number of libraries for data analysis but slightly less frequently used compared to Python.