I loathe this saying, along with the entire "explain it like I'm five" trend that has taken off in recent years. Yes simple and concise explanations are valuable, but there still needs to be a baseline. There are certain things that just cannot be explained to a layperson unless they have some prerequisite knowledge in the field. I didn't know Einsten's grandma, but it is safe to assume that the lady did not exactly grasp the theory of relativity or quantum physics.

Here's a better quote (also sometimes attributed/misattributed to Einstein) – “Everything should be made as simple as possible, but no simpler.”