It's more about what cold meals stereotypically are--some processed cereal, two slices of wonderbread with some bread and ham in them--that drive that rhetoric. Sure, a cold corn and arugula quinoa salad is certainly much healthier than the vast majority of meals, hot or cold. But on average hot meals are likely slightly better.
This comes up a lot in political discussions of child hunger and school lunch programs. "X millions of children don't get even one hot meal a day!"