Any module with a UI is valid to consider improving the UI/UX. I would recommend that you pick one that is interesting to you and explore it and research it and then come back with a proposal as to how it might be improved. This is probably best doen through an enhancement proposal ticket on the repositiory. If you proposal is positively recieved by the maintainers then you can go ahead and work it up and submit it as a pull request.

There are many different aspects of the system and each individual engages best with different parts of the system. So pick the part that is most interesting to you. Starting small will help you get your bearings and learn about the software engineering processes we use in the project. After that you can grow the scale of your contributions. If you try to do very big changes early it's likely to end up with frustration due to following different processes, even if the technical goals and results are likely similar.