Having a mentor is not necessary, but can definitely be very helpful. Not only with trading, but with anything in general. Someone giving you good advice as a beginner is super beneficial if your mentor actually knows what he's/she's talking about.

The only thing is that it might not be easy to find a good and effective mentor, a mentor that's actually there for the purpose of helping you, and not leech off you or something. Especially knowing that you will most likely be looking for one online. I'd definitely prefer a person I know personally.