On its own, Frosmoth won't do much. But with a good partner card, it can be a real force to reckon with.
Play it with Quagsire or Lapras V, drop the water-energy onto Quagsire, and move it to any other Pokemon that is ready to attack for some extra water damage.
In general, almost any water deck would want Frosmoth in it. It's just an excellent way of distributing that extra energy.