Was Mary sinless according to Catholic teaching?

Yes. Catholic teaching holds that the Blessed Virgin Mary was preserved from all stain of original sin from the moment of her conception — a privilege called the Immaculate Conception — and that she remained free from all personal sin throughout her life, by a singular grace of God in view of the merits of Jesus Christ.

Scriptural Basis

Elizabeth, filled with the Holy Spirit, calls Mary 'blessed among women' (Luke 1:42), a superlative that further underscores her singular holiness.