Nakoruru has a lot of things going for her. She's one of the most popular SNK characters period, having crossed over in both SNK Heroines and KoF, is currently the leading female in SamSho. She was recently ranked third in a popularity poll, below only Terry and Geese.
While Smash usually goes with "leading character" reps, SNK's characters are usually spread so far and wide, only for them to congregate under a single banner in KoF, that the amount of characters they have which constitute leads can be pretty wide. There's also the fact that in recent history, all of SNK's cameo appearances have been different characters. DOA6 got Mai and Kula, FELX got Terry, Tekken 7 got Geese, Million Arthur Arcana Blood got Iori, etc.
Right now, Nakoruru is one of the only "iconic" SNK character who hasn't crossed over with a non-SNK game yet. Kyo, who is KoF's "poster boy" if anyone, is also another candidate. That said, the former also represents SamSho which is currently active as well, and the pass is lacking female characters, so who knows. If SNK got to be selective of who to include, it's possible they would want her to set apart from other cameo characters.
Either way, whoever it is, is most likely someone who's been in a KoF game, given that it's SNK's premiere series and also a franchise that was very formative for Sakurai.