It's as safe as the people using it, so yes and no. If she's not meeting random strangers then I'd say it's perfectly safe* but it is primarily a dating app so most of the people using it will be looking to meet. So it just depends whether you trust her that she is just using it to make friends (you know her best but I'd be really surprised if many people would really chose to use tinder for that, it sounds like something you'd just say to your parents tbh
)
Edit: *Although actually there's also the dangers of sending people intimate pictures and those pictures being used to bully them online which happens a lot especially with people around her age.