Heya Majaa, welcome to the insanity Dreamviews forum!

It depends on what you mean by "learn." I have 'learned' some new runs on a piano because of a jam session I had in a lucid dream. I am also sure that you might learn a bit about yourself by the things you decide to do in a world where there are no consequences.

So, if you are referring to learning/improving skills and/or abilities, yeah it is possible. Not only can you generally be more creative in a dream, but you can build muscle memory as well.

If you are referring to life lessons, I can't imagine you could learn any NEW information, but perhaps new conclusions built from old information more obvious in the dream. Though, honestly, I am not really sure.

EDIT: One thing I forgot about, some people can use lucid dreams to learn how to conquer certain fears by dealing with them in the dream. So that is also something.