I'd suspect that it can be helpful because it distracts you from thinking about other things, and if it's a show you're familiar with (or something where you don't need to pay too much attention) then it doesn't require much active effort from you either, hopefully leaving you in a state where you've purged yourself of too many other pressing thoughts and you're relaxed - both of which should help you sleep.

A book might be better though, since blue light is supposed to promote wakefulness and is more prevalent in light coming from screens that from your average light bulb.

(FWIW - I tend to watch cartoons on my iPhone just before I go to sleep, but only ones which I find slightly amusing and wouldn't watch otherwise)