The issue here is even more fundamental - the lack of any Graph API endpoints for Exchange Online administration. Microsoft is yet to release any information on that front, so for the time being you need to rely on other methods (ExO PowerShell or good old MSOnline module). I suppose you can also try /messages or /mailFolders endpoint against a given user object, but that returns a 403 in case no mailbox exists, so it can be misleading.
We need to be able to distinguish between shared mailbox and normal user via Microsoft Graph!
At this point, the user that is being retrieved by Microsoft Graph doesn't have any properties that would make it clear that this user is a room or a shared mailbox. But for our product we need this information! I've would rather write this article as a suggestion on your uservoice site but it "is not currently active" so I expect that I can get an answer for my question here.