Do we have any user exit or BADI add serial number to the reports like Monitor Production/Planned Orders ; Display Material Document list
We don't have User Exit concept with Public Cloud. If you are looking for any BADI - i would suggest to have a look at the SAP product help for BADI in case if that suit your business case.
At the same time, You can create custom CDS views from two standard CDS view (With Cardinality between two CDS) and if exist the Serial number field and it is enabled you can get that as part of Custom CDS view. You can use the same as report - as such we don't have any reports and everything is around using the Fiori App.