I don't see a reason why it should be a monero address tied to it. Just make it a random string like mullvad does)

The nature of Monero address is public (it can be used publicly to receive xmr), and you don't want to use a public string as a secret password. Practically, though, if it's possible for you to keep it absolutely secret and safe, you're free to do so at your own risk.

If it's the main address starting with "4" and later you happen to decide having fun p2pooling using the same address, then obviously that's not good. To avoid unnecessary worries, perhaps making it a random string, like @Unkn8wn69 said, is a good idea.

Technically, since the string length of a monero address (hence the name space) is finite, it's not guaranteed to be unique, though the probability of collision is vanishingly small and this won't be a real concern at all.

Its a good idea.

If the service supports delegated user accounts (some permissions but not full account access), it might not work