I agree with his level 1 being rather weak but his level 2 I see as being decent. His level 1 is indeed quite weak as it's limited to one kill per use but at least it's very quick to reach the target (hard to dodge). His level 2 has very good AoE after the hit, basically kills anything grouped together once it hits something. I've seen many vids where Sweet Tooth just activates his level 2 beside an opponent and boom, 2 quick kills without even controlling the rocket. His level 3 is quite good too, the only downfall is the short time period of use.
Also, for those who haven't see it yet: