i get where you're coming from, but i have to say you can tell how bad someone is from a video, just it's much harder to tell if they're merely ok or great. I mean you can definitely judge how a person reacted to certain things in a video, for example laser camping by fox. Or as a marth player i can definitely judge a poor approach from a good one, mindgames are harder to see, but if you play a match in slow mo and think about it from both perspectives you can definitely see a lot of that too. I tend to watch certain sections of pro matches in slow mo as i try to figure out why the two players made the decisions they did.
another thing you can definitely judge mistaken calls, when fox run in and rushes a marth from above with no concern for the fact that marth has insane priority above him, then you know that the fox is a n00b, and any calls of how that blind charge is a mindgame are thrown into question because a good marth player would just swipe them out of the sky on reaction.
Anyways, i find it extremely easy to tell the players who are substantially worse than me, and extremely easy to tell how much better the pros are at reading and mind games compared to me. I think the hard place to judge is when people are at near the same skill level.
However, i agree with one thing; the greatest flaw in videos is how hard it is to tell whether the player was taking it serious or not...sometimes i fall asleep in friendlies if it's like the 40th match or something