Your girlfriend “stalked” your ex and watched all of her YouTube videos?

Do you think that’s a bit much?

Depends how many videos and how often the guy was in them. Plus how long their relationship was. It's not crazy to be curious to see what things your SO likes in another person.
I was in the videos for about 7 months, id say 10-15 videos. She had the channel from before that. Issue is they were relationshipy videos, like a viral tiktok “prank” was just kissing your partner all day and she did that and put 10 mins of us just making out online. My gf 100% hated that, but also why watch it then???

My gf 100% hated that, but also why watch it then???

I’m glad that you’re asking yourself this question.

Unfortunately, the answer is something like: She was looking for something to use against you.

That in itself is not healthy behaviour, but what makes it even worse is that once she found it, she went snooping through your laptop and found something else to weaponise against you.