Fast answer
Frostnip is a good Tatari to build when your team needs Water damage and extra pace control, especially in messy mode fights.
What Frostnip does well
Frostnip is valuable because slowdown is real utility. In launch play, buying time is often just as useful as adding raw damage.
Evolution priority
Good, especially if your account lacks Dewgrub and wants a stable Water option that helps both progression and mode play.