There shouldn't be any bullet drop when aiming downward...

I was just going off range, not angle. You're going to see slight drop at 150-200 yards. But, yes, angle changes things a bit.

When a rifle is zeroed...

Correct, but that's a whole other issue, because you could say there's zero bullet drop at 2000 yards, if zeroed to that. Point is, at 150 yards, you don't even need to zero, because drop is negligible. Of course you still should zero, but the impact of bullet drop isn't much either way.

Regardless, pros don't go for head/neck shots. They aim for center mass.

I haven't confirmed, but apparently Charlie Kirk was wearing body armor, and the shot actually did hit center mass, before sadly ricocheting up to his neck.