Fingerprint protection for the app isn’t a security feature so I’m not sure why you’re raising this here.
But the app is kept secure firstly by your device’s passcode protection & secondly by requiring additional authentication i.e. enter your card’s PIN or CVV, before you can remove money from the app.
As long as you follow the T’s & C’s, Monzo is liable for any theft of your money so it’s in their interest to keep the app secure. Therefore, the fact that they haven’t added PIN protection for the app itself already suggests that it is..just because other apps use this protection doesn’t mean that it’s the only way to keep your money safe.