The TapeAlert for cleaning is generated by the tape drive, not NBU.
So, if email alerting is needed, you need to find out if the tape library has that kind of capability.
NBU will only know about the TapeAlert when it needs to use the drive.
So, best practice is to configure automatic cleaning by having one or more cleaning cartridges in the library.
Please have a look at this solution from @mph999 (ignore the Symantec links - they don't work anymore) :
Technically 'library based cleaning' is not supported, although people do use it, it is down to the library oif it is able to send emails, although if it is working, there should be no need to do anything other than check the cleaning tapes have not been used up every now and then.
If NBU is used to do the cleaning the same applies - you will know from experience how often the drives need cleaning, so can estimate how long a cleaning tape will last so know approximatly how often you will need to insert new ones - just throw a few tapes in the library and leave it to it.
You can parse /usr/openv/netbackup/db/media/errors (or same file on Windows server) and send an alert if you get Tape Alerts like:
0x14: 'Tape drive needs cleaning NOW', 0x15: 'Tape drive needs to be cleaned soon',