Marandahir
Crown-Forester (he/him)
If he does so often, yes he does have to be evil. Says so in the PHB - 'ONLY evil casters do so regularly'.
That's not open to debate. If you regularly animate the dead, you're evil. Says so in the PHB. Animating them is never a Good act (no matter why or for what reason you do so).
Whether that's a metaphysical consequence of using 'dark, foul unholy' magic (as described in the MM under the Zombie and Skeleton entries) to animate them (same as how a Jedi using the Dark Side in Star Wars gets corrupted), or a philosophical consequence of the choice to use 'black magic' to animate and bring into existence evil baby eating monsters powered by unholy dark energy, by defiling the corpse of a deceased sentient creature, is up for debate.
Barring DM intervention to say animating the dead is not, in fact, evil, you cant be someone who regularly animates the dead in 5E and be Good aligned.
What's more evil, the act or the motive? Can the ends justify the means? I wouldn't be so fast to deal out moral judgement, when you don't know this Necromancer and their purposes and what ends they'll come to. I imagine only the Necromancer can truly know what's in their heart and actions.