Am I stupid if I start off as 400 ELO after watching chess vids and analyzing a few games?

Everyone has to start somewhere. A few vids and a little analysis is just a small step. A 400 rating right now is nothing to worry about.

No, completely normal. I started of not to long ago, managed to increase to 700 by mainly just trying not to blunder to much, control center, develop etc. I can recommend "building habits" by chesbrah on youtube. He has a series that you can follow and it works, went from 300/400 to 700 just like he does in the series.

It’s not about where we start, it’s about the journey.

Your rating after playing a short time doesn't mean anything. If you want to become a good player, you need to play a lot of games. And since you are a beginner, you're going to lose a lot of them. If you really like chess, and you work on improvement, your rating will reflect your improvement. Don't sweat it!

On the other hand, you have to quit playing like this! Almost every blunder you made was a one-move blunder that you could have easily seem if you were paying attention

Nothing is bad for a beginner.

However, you started in February. That was 3 months ago. 29 games, 0 lessons and 0 puzzles in 3 months? That is not a lot.

When you first start playing chess, it's a challenge to see where the pieces can move to and which parts of board they control. Let alone actually play a game. When i began I knew I was terrible, always missed attacks on my pieces and generally just blind to a lot. Being crap is what motivated me to improve. Everybody is bad when they start barring some inhuman prodigies so try not to dwell on it and instead focus your attention on improving.

You are doing great and chess is hard.