The title did not start being used until the early third century, so I imagine referring to him merely as St Peter was well established.
Also, sainted popes are not invariably referred to as “(St) Pope Whoever”. For example, the SSPX is the Society of St Pius X. A parish church near me is just called St Pius X.
On the other hand, in Catholic circles, I have rarely heard of Our Lady referred to as “St Mary” – usually instead by one of her many titles, or just as “Mary”.