The chance for Pickup to activate after winning a battle is 10%, and the maximum chance of a Rare Candy being "picked up" is 10%, which means that there is a 1% chance of getting a Rare Candy from a Pickup Pokémon. Even if you have a party full of level 41+ Zigzagoon, the chance will be 6%.
So really, it's probably not an effective way of farming Rare Candies. It is the only way though, as far as I know. Rare Candy isn't a really great item anyway, it is quicker to experience grind at the E4.