Should "Reset settings" or "Reset to default" be disabled if the user has not made any updates/changes to default settings?
The other option being leave it enabled and then tell them nothing changed? - Annoying. Leave it enabled and do nothing? - Lack of feedback can drive people mad.
Users basically need to know what they can and cannot do, and generally won't appreciate you wasting their metabolism.
However, it ALWAYS depends on the larger context. There are cases where you allow users do something that 'won't work' - like this site allows you to upvote even if you are not registered, only to tell you you need to register. There are better examples.
Seeing a button and realizing it is disabled, thinking about under what circumstances it might become enabled, and remembering the supposed functionality are small, but unnecessary tasks for the user.
Since the button's action if clicked can be understood as "set the whole form to its initial state", there's no reason to disallow users to click it while the form is already in that state. No harm is caused by resetting an empty form, and the user doesn't have to pay attention to the mechanism of enabling the
reset button when input is given.
If the button was to refresh the site for example, and users are expecting that behaviour, then disabling the button would unnecessarily obstruct the user from achieving this side goal. While this is a minor issue to add, it might be important, depending on your user demographics.