(Spoiler) Why Lyse have so bright skin ?

[Iscah]

It's quite possible Lyse wasn't originally planned to be Ala Mhigan, and her character design was already locked in at the point they decided to change that.

She's also spent a long time away from the country - first in Sharlayan, then mostly in Gridania.

Also, Minfilia is Ala Mhigan as well, and she's not quite as pale as Lyse but she's not dark either.

When playing through heavensward i haven't found any dark skined ishgardian so i kinda hard for me to imagine that ala amigo is so hot and lyse have very white skin.

Look again. From the MSQ we meet Grinnaux and Charibert, and it also seems to be a family trait of House Durendaire, most prominently Jannequinard from the AST quests. (Also their ancestor in our Echo-vision of Haldrath's knights, so it's not something recent.)