I don't think watching tv dumbs kids down, and in fact my ds has picked up loads from ceebeebies in terms of knowledge, vocabulary, numbers etc. and there are some programmes he really loves that have enriched his imagination and playing/storytelling etc
However, with my ds, tv watching had crept up a lot when I was pregnant and with a newborn, and had stayed at a couple of hours a day as that was what he had become used to, and I found his behaviour generally had got more erratic, he seemed more stressed and whiny and less able to entertain himself as a result.
We were doing lots of other activities and getting out as much as possible too, but he was reluctant to get toys out, didn't want books, could only wind down with the tv.
I took drastic action and cut him down to half an hour, at the end of the day while I'm making dinner, and for the first day or so without it was hard work filling the time. By day three though, I no longer had to fill the gaps between activities for him, as he was playing really well on his own again without prompting, finding things to do for himself and bringing me things to read/play with and he seems much calmer and happier too.