The randomly generated username you see on your Account is what we call a “nickname.” It’s designed to keep you anonymous. It shows up publicly when using children’s experiences such as LEGO Life. Your nickname can only be changed once every thirty days.
Since LEGO IDEAS is for members 13 and older, you can change your display name in your profile. It defaults to your nickname initially, but you can change it to what you like. Your display name is only for LEGO IDEAS, it does not show up other places you use your Account.