Yes. Big fat, YES
Now the reasons for this are obvious. While Ralts by itself is weak, it evolves into a very strong Pokemon:
Gardevoir: which is helpful in-game as well as competitive with its high Special Attack, and decent Speed. It makes for a good Psychic type in-game with massive battling benefits. What's more, with that awesome Mega Evolution, it is definitely a force to be reckoned with. Learning moves like Psychic , and gaining STAB on it, makes it very appealing. Learning Shadow Ball and Thunderbolt is just icing, as this Pokemon will undoubtedly help in the Pokemon League.
Of course, there might be better Pokemon out there, but they take far too long to evolve, and the Pokemon is question is definitely still worth it, specially if you lack a Special Attacker / Psychic type on your team.