Damaging to a baby's development.
You won't find any irrefutable proof of this. You will have people who tell you it's madness to say such a thing, and people who tell you it's common sense to say such a thing.
You will find studies that support both sides (as with every topic imaginable) and you will find a consensus view.
It's up to you which you find most convincing.
There are many things that are "bad" as well, foods, habits.... and again you will not find agreement.
But even something that is bad, for a short time, or in small quantity, is going to cause such minimal damage that worrying about it would be unnecessary stress. I think "20 mins here and there" is such a case.
When it comes to co-parenting you definitely want to pick your battles. If your child is safe and happy (not in immediate danger of harm, and happy in herself) then not only will your partner probably not listen to you, but professional bodies would consider you as being controlling over your ex's time with the child.
You'll just have to let him parent how he wants as long as he keeps her safe. And if you think it's bad to be in front of the telly then you can certainly mitigate any damage by having no telly at home.
There really is no reason a child NEEDS to sit in front of a screen for long periods.