Your own question contains the answer
If by "perfect" you mean "sinless", and that the Immaculate Conception as the Church teaches means Mary was conceived immaculate in the womb of her mother i.e. without contracting original sin, then her Immuculate Conception was just the beginning.
Think of Eve [and Adam] created sinless and in innocence until they fell.
In CCC 493, the Church teaches that by the grace of God Mary remained free of every personal sin her whole life long, and therefore from your taking sinless to mean perfection, Catholics can say mary was perfect all her life long because by the grace of God she remained sinless all her life long.
Please note that all of us are called to be perfect as our heavenly Father is perfect except our path unlike that of Mary is by forgiveness of our sins and our purification from them.