Tantrums are a normal part of development. Too many might be a behavioral problem, but it is flatly not true and flies in the face of all childhood development studies to say they should never happen.

I have observed, as many other parents have as well, that if the kids spend too much time in front of the screen (> 20 minutes, give or take), they become resistant, moodier, and worse behaved. This is not true if we're playing a board game or having reading time. So, I think a bit of moderation is important.

Incidentally, since having kids, I've also noticed that I am short-tempered if I spend too much time in front of the screen, a good reminder to take a walk a few times during the day.