I think it is a zoning ult, yet even so, there are a few problems I find with the ult in its current state:
it’s blocked by obstacles due to being a line of sight ult. Even throwing it in a room can mess up due to obstacles, stairs, walls, etc.
It has a longer charge than most dps ults due to Mei’s poor dps.
It takes roughly three seconds from the moment of Mei announcing the ult for it to actually activate (cast time on Mei throwing the ult+air time+cast time on Snowball activating). This can be circumvented by throwing it at your feet, however, it always gives plenty of time to escape, and to focus the Mei before she can apply the primary fire freeze, and even then, her primary fire doesn’t pierce, you have to apply it one at a time to each enemy.
It is absolutely countered by jumping. They could easily change this by giving it the same slow effect as her primary. Additionally, most heroes in the game have some form of escape ability, and even those that don’t have mobility still stand a good chance of escaping due to the cast time.
It suffers the same penalties as a Zarya ult without the same strengths. It can be eaten by D.va, deflected by Genji, and can be lost even when cast if Mei dies while snowball is still airborne. The only advantage over Zarya’s is that it charges faster (still longer than other dps, however).
I don’t think the ult needs a lot, as it is good at holding a point, but I think it needs something to help it, slightly less delay on cast, added slow effect, removal of some lOS restrictions (as they did Sombra’s). It’s an ult, so should still feel like an ult considering its significant amount of counter play.