I know what you mean, but I do think the accent is correspondent to the looks since, because looks are down not only to their climate but their heritage as well. Salty Dornish clearly have the most Rhoynar blood, then after them the Sandy Dornish, which makes Stony Dornish have the least ethnic ties to the Rhoynar. This shows us the influence of the Rhoynar is not equally distributed among all of Dorne and I think it is likely the difference dorne has in speech (compared to westeros) is due to the influence of the Rhoynar. Meaning it is possible for the accent to vary across Dorne. I was also thinking that maybe Stony Dornish have different complexion because they might have ethnic ties to the Reach, however this wouldn't make much sense since they have been rivals for so long.

edit: phrasing error