Unlike nearly every other unevolved Pokémon in Ash's possession, there's actually an in-show reason for why Rowlet never evolved. Ash's Rowlet is a little bit of an air-head and accidentally swallowed an everstone, a special item that prevents Pokémon from evolving.

In which episode does Ash's Rowlet evolve?

In No Stone Unturned!, Rowlet was used in Ash's battle against Hau, where it went up against its evolved form Dartrix. Rowlet's tendency to fall asleep caused trouble for it, and it was eventually defeated when Ash ended up dropping his Grassium Z while trying to use it.

Does Rowlet evolve in Pokémon?

Rowlet evolves into Dartrix, who evolves into Decidueye.