Definitely. They love candy, and it's up to the trainer to make sure they don't eat too much of it, or else they can suffer nutritional issues like I mentioned above.
An overly spoiled Ralts can be even more difficult to train than usual. It's an important skill for the Ralts line to learn to tolerate emotions, both its own and the emotions it picks up from surrounding people and Pokemon. Ralts are often emotionally overwhelmed, especially when in more crowded cities. They learn to tolerate these emotions as they grow up and gain more fine-tuned psychic control.
But if you give into Ralts' tantrums, you can stunt that emotional growth, leading to behavioral issues.