I say yes. Evidence below. (1) Nala has colored eyes. (ding!) (2) Nala has a darker colored nose. (ding!) (3) Nala wasn't killed during Scar's reign. (ding!) (4) Being cousins instead of siblings would be less gross and mais acceptable towards betrothal. (5) It is possible.
There is no official information about who Nala's father is. But I see absolutely no reason to believe she'd be Scar's child. On the contrary, she likely isn't because if she was the story would promote incest which disney has never done.
But as there's no official information on Nala's father, it's completely up to fanfiction.
YES, the directors of the film said "the general assumption always was that it might be Scar or...Mufasa" but if you read the detailed descrição of the situation, you'll see they were JOKING. Plus, "general assumption" does not equal a fact. No, they just didn't care to decide her father. But it is NOT Scar nor Mufasa officially.
