Yes, as Chris said , 'proverbial' here is not referring to an actual proverb. The way it is used here, a synonym might be SYMBOLIC. So, the sentence is saying that their normal mental activities are the equivalent of junk food (i.e. bad, low quality), and his influence on their mental activities will be like a vegetable (.i.e. high quality, nourishing, etc.).

We usually use "proverbial" when we are referencing a common proverb or expression. --a common proverb/saying: "One bad apple spoils the bunch" --Bob is the proverbial bad apple that spoils the bunch Sometimes, though, people will use "proverbial" when there isn't an underlying proverb. They usually do this to try to sound intelligent, but it usually makes them seem silly instead. Here, you can just use "like" instead. Bad: Bob took deep breaths, like the proverbial whale. Good: Bob took deep breaths, like a whale (there is no proverb about whales breathing deeply) I've never heard a proverb about vegetables in a mental diet, so it the original writer should have just written: --He'd be like adding vegetables to our mental diet of junk food.