The Greek phrase is definitely τῷ ἁμαρτωλῷ = "the sinner", but this does not necessarily compel the translators to translate thus.
However, Ellicott has this:
God be merciful to me a sinner.—Literally, to me the sinner, as
though, like St. Paul, he singled out his own guilt as exceptional,
and thought of himself as “the chief of sinners” (1 Timothy 1:15).
Similarly, the Cambridge Commentary has:
God be merciful to me a sinner] Rather, O God, be merciful to me the
sinner.The word for ‘be merciful’ means ‘be propitiated’ as in Hebrews
2:17. He speaks of himself as the chief of sinners, 1 Timothy 1:15.
Also Jamieson-Fausset-Brown Bible Commentary
be merciful—"be propitiated," a very unusual word in such a sense,
only once else used in the New Testament, in the sense of "making
reconciliation" by sacrifice (Heb 2:17). There may therefore, be some
allusion to this here, though not likely.
a sinner—literally, "the sinner"; that is, "If ever there was one, I
Personally, I agree - this man is in no doubt that is not just any sinner but a great sinner - THE sinner. In the absence of the article, the Greek simply means that the man considered himself in the category of sinners. However, the force of the article here is to make him the great sinner with particular sins that specifically need repentance and mercy.
Thus, Jesus could follow this with: "I tell you that this man, rather than the other, went home justified before God."