According to the Oxford Jewish Study Bible, the Hebrew of Jeremiah 6:2 is "uncertain", perhaps due to some ambiguity on how the original Hebrew text might have been vocalized to create the Masoretic equivalent (someone schooled in Hebrew can comment). The underlying Jewish translation upon which the OJSB is based renders the verse similar to what the NASB chose:
Fair Zion, the lovely and delicate, I will destroy
The Septuagint reading here (dating to around the 2d century BC) is:
καὶ ἀφαιρεθήσεται τὸ ὕψος σου, θύγατερ Σιων
which translates to something like:
And your exaltation shall be taken away, Daughter Zion
George Lamsa's English translation of the Peshitta, itself in turn a 2nd century AD translation from Hebrew into Syriac, reads:
I have likened the daughter of Zion to a comely and delicate woman
which matches the KJV.
Finally we have Jerome's translation into Latin from yet another proto-Hebrew text, around the 5th century:
Speciosæ et delicatæ assimilavi filiam Sion.
I have likened the daughter of Sion to a beautiful and delicate woman
At least part of the ambiguity seems to come from the word דמית that appears in the verse. In the Masoretic Text, this is vocalized as דָּמִ֖יתִי (damah), which means "resemble."1 Emmanuel Tov2 notes, however, that something closer to the Septuagint reading can be obtained by interchanging ד with ר, which could render the Masoretic רְמִיָּה (remiyyah), meaning "deception" or "fraud"3
This doesn't quite explain the ambiguity, but perhaps it is enough to suggest that either there are different proto-text manuscripts or different ways of vocalizing the Hebrew that lead to the differences.
In short, I don't think there is a way to determine which is the "correct" translation. The oldest manuscript (Codex Vaticanus Septuagint, 4th c) agrees more or less with the NASB, but it is a translation from Greek, supposedly dating to the 2d century BC. The alternate readings (Peshitta, Jerome, Masoretic Text) are from more recent proto-Hebrew texts, but I think it is impossible to conclude which is closer to the "original".
1. See, e.g., J. Swanson, Dictionary of Biblical Languages with Semantic Domains: Hebrew (Old Testament)
2. *The Parallel Aligned Hebrew-Aramaic and Greek texts of Jewish Scripture
3. See, e.g., W. Gesenius and S.P. Tregelles, Gesenius’ Hebrew and Chaldee lexicon to the Old Testament Scriptures