Probably going to be a major unpopular opinion but coming from a person who was 13 not majorly long ago - I was the exact same in school holidays and I was fine. I was talking to my friends, watching YouTube, reading up about things I found interesting, finding new tv shows to watch, trying to write stories, all on my phone or computer.
I just genuinely didn’t want to go out if it wasn’t to do something particularly fun (like outing, meet friends etc). I was happy to just laze around. Everyone I know was like that as well, and everyone’s parents didn’t like it, including mine.
Yes it could be the precursor to or a symptom of something worse, but don’t take the constant screen time as the sole indicator of that. Lockdown is a naturally depressing time, but any thing out of the ordinary in regards to mood, behaviour, emotions etc is obviously a red flag.
In the absence of that, I honestly wouldn’t worry too much about its affects. Ideally she does need exercise and fresh air, but this isn’t permanent. I personally would not worry too much, but I understand why you are