It is bad for a toddler to watch TV all day because they are sitting around which is not healthy for the toddler and also the light from the TV can be bad for the toddlers eyes as well.
However it's not bad for a toddler to watch some TV but it should not be all day.
I let my 18 month old watch TV.
It's okay to let your 18 month old watch TV as long as they are not watching TV all day.
There's nothing wrong with allowing a kid or toddler to watch TV some as long as they are getting away from the TV for most of the day to do other things.
Also have the toddler watch good educational cartoons.
Although it's also okay to allow the toddler to watch other cartoons even if they are not educational.
I watched TV at 18 months old and as a toddler and child and I turned out just fine.
TV is not bad for toddlers as long as they are not watching the TV too much.
The toddler needs to be away from the TV for several hours and out playing when possible or playing inside the house away from the TV.
The TV should not be a babysitter for the toddler or child.