Nikon d5100 filming tv screen quality?

I just want to know how a video of a tv screen while it is on would look using a d5100 on a tripod

It depends,
I looked at the specs and it has a custom setting for filming.
Oh and tube or a flat TV?

First do your homework on terms. You can't "film" anything with a digital camera because it doesn't use film - it shoots video.
Answer: Terrible because the frame rate is different and you'll get very nice looking black frame bars running through your video.

Jim goes to theater to watch a video, while everybody is watching the film.
Answer: yeah, it will look terrible, especially if it's a CRT TV.