I think it's deliberately unknown whether Nettles has Targaryen ancestry. In a way it highlights how it's unknown whether one needs Targaryen ancestry to claim a dragon (which is why she's central to arguments about whether non Targaryen's can or not). It even builds tension about whether non-Targaryen(s) is/are going to claim one of Daenerys' dragons.
Sure Nettles doesn't look Valyrian but neither does Rhaenys Targaryen (or Orys Baratheon for that matter), and crucially and infamous neither do Jace, Luke, and Joff. After all, while their father is disputed, mater semper certa est: the mother is always certain. So while Targaryen dragonriding magic might exist, it's certainly not stored in the silver hair or the purple eyes (or the culturally constructed patrilineal surname).
As an aside Nettles is described differently from Summer Islanders but that's probably because she's noticeably mixed race (not that they have modern categorisation of race in Westeros/Planetos), especially as iirc there are no other ethnicities that are described the way Nettles is.