Not being a frequenter of the Regency world, I assume that the thing in the title would be a good thing to be; however, depending on the passage, it is difficult to tell. Does anyone know whether or not it is desirable or undesirable? If so, would you please share and mitigate somewhat this one's ignorance?

Oh, yes, Steven, being a Pink of the Ton is being the top of the top. The best in Society. Though I think it has just a hint of sarcasm--being a pink means being very much IN FASHION, though largely as a follower, not as a trendsetter.
Those who are the top of fashion, yet stand above it a little are Nonpareils, or Nonesuch, or Corinthians (these are most the sporting guys).
This might be useful:
http://www.thenonesuch.com/lexicon.html
Dear MamaT,
Thank you!
Steven