It sure does! Plus, Pokemon that you keep from evolving learn stronger moves much faster! So that kinda balances it out! Usually I let them evolve if it happens just through training, because that way I feel like they see it almost as a reward for their hard work? But if they evolve through special conditions I like to let them decide! Like, my Eevee chose to become Espeon, or my Kirlia chose to become Gallade...

Magikarp into Gyarados, though, for example? That was just a happy accident.