Quite possibly, but they have been fashionable amongst teens for 5+ years, and if not used as a school bag, really works well as a gym bag, a holiday tote etc. frankly, mum can start using it if it gets abandoned after a couple of years, at which point it will still be perfectly serviceable.
Plus, it’s her birthday present- how long do you expect a birthday gift to still be popular and how much use do you expect it to get? It’s a lot of money to spend on a school bag if you are buying the school bag additionally to any other birthday/Christmas gift- it’s not really a lot of money to spend on a teens main birthday present.