It depends what one wants to do. When connected to the CDB, Spotlight can monitor the CBD’s performance and the resources using by each PDB. This needs a common user and the objects will installed in the CDB.
When connected to the PDB, Spotlight will just monitor that specifies PDB's performance; Spotlight user could be a common user(Objects in CDB) or a pdb user(Objects in PDB).
If user is using PPCM(Spotlight Predictive diagnostic), it is recommended to create Spotlight user in each PDB, since each database's data is difference.