This is a complicated subject and the usage area defines what rules apply.
Typically in the netherlands:
Mostly, yes mandatory, even if not functional as noted in the rules. "if the generator is used a IT (no earth pin) then they will not act"
Exceptions for using generators as IT-system are the amount and class of devices attached and the length of cable. One class I or some class II, within 30m and less then 3kVA.
Above 18kg isolation monitoring is even mandatory when used on building sites.
In any case all metal structures or objects need to be bonded when the generator is to power them to not have potentials between them.