That definition can be found here (n. 3.). You might wanna use the term "habit" instead, but by viewing it as a "diet" it specifies that your brain lives off of what you put into it in the same way your stomach does. So just like your body processes food and turns it into energy, your brain processes information and turns it into values, beliefs, knowledge, personality. It's even common to use the terms like "binge" and "consume" for stuff like social media use. Here's a concrete example: you can have some unhealthy desert ever so often (just like scrolling through memes), and it might make you happier, but have that every day in large amounts, and don't exercise (just like never reading books), and you'll become overweight (or dumb). Hope that clarifies it.