Unfortunately that isn't something that can be done as of right now.
The user name currently must be the token provided by MxId, e.g: https://mxid2.mendixcloud.com/mxid2/id?id=4e7ac3ef-d473-457d-9ca0-741cffec7cf4
You do get the user's display name / full name. But that is all the information you can currently request from MxId.
If you enter a ticket explaining which kind of information you would like to retrieve through the API we can put this on the roadmap.