It's more of a nn for me, but it doesn't have to sound like the name, like
Ace could be for someone actually named
George Nathan. It sounds more like a friendly term.
That said, if you really liked it, it could be their fn. It's short, so I would pair it with a longer MN, like:
Ace MatthewAce GregoryAce EdwardAce FrancisAce Nathaniel
Ack, it's a really hard name to pair!