Though Tally doesn’t elaborate, it’s likely that kisses between uglies “don’t count” because they’re not considered real people yet. When this one feels like it counts, then, it shows that Tally is starting to come into herself and value herself for who she is as a whole, “ugly” person. Destroying the pendant, meanwhile, is something Tally seems to think of as a gesture of support for Shay. Even if Tally accepts and reciprocates David’s affections, she still essentially agrees to respect Shay’s choice to come to the Smoke by destroying the chance that Special Circumstances will follow.