You have to query the application/service principal object itself to get that info (resourceId value if starting from the AppRoleAssignments endpoint). For example, the below query will give you the list of AppRoles defined on a given service principal, including their displayName
GET https://graph.microsoft.com/v1.0/servicePrincipals/837cf7bc-bc22-42eb-8c3f-375a6372563f?$select=appRoles