On the contrary, people were thronging to John's Baptism, including Jesus the Christ Himself. It was the in thing to do in those days.
What is the more likely assumption to make is that the process was not complete for mankind until the Lamb of God made His Sacrifice.
Since John's Baptism was such a phenomenom in those days, why would we speculate that Dismas was also not Baptized? Because he was a thief? I say we have Baptized theives among us even today.
We are however, certain of a few things; namely that he is now in heaven because Jesus told him so and that Christ did not establish the necessity of Baptism till after His ressurection.
Since Christ did not institute the Sacrament of Baptism until after His ressurection, whether or not the good thief was actually baptised or not is of no consequence.