As long as you do not reveal your bank account credentials to any known / unknown person even by default, it is always secure. Although you think that your mobile is secure, it is always advisable to keep your credentials safe and keep changing your mobile wallet app and banking passwords frequently. Otherwise, it is safe using a mobile wallet app because you can instantly know what is happening with your account if online purchase is made.
Ensure that you register your mobile number for banking purposes with the bank so that when you use a mobile wallet app, it will not trouble you much. Also, due to the generation of a one-time password every time you make a purchase, your bank keeps you always in the alert about the financial transactions happening with your account.