The APP attribute seems to always be a problematic one!
Here's one problem an answer for which I have been unable until now to find on this forum:
While I have to admit that I have never actually researched the subject of different standards of beauty among different cultures around the game period, I am going to make an educated guess and assume that they existed. Cymric folks know that all Pictish women are ugly etc. but I don't think the Picts quite agree with them. One who might seem like the epitome of beauty to some might looks sickly or deformed or just plain ugly to others. Even, I believe, if we are only considering the myriad cultures existing within the British Isles and western continental Europe, and not completely foreign ones that might make things all the more complicated.
In short: in your opinion, should the APP attribute be universal? One reason for it to be (indeed, perhaps the main reason) is that it makes things far simpler, and most of the time in games (tabletop especially), simple beats realistic. But supposing we are chasing a greater degree of realism here?...
Would you suggest to even try and create some alternative rule? Leave it to the GM's arbitrary decision? Just ignore it and have fun?

Food for thought and so on and so fourth.

Thank you very much,

A humble stranger

All the other attributes are universal, so why complicate matters unduly? Distinctive Features exist so you can call attention to any specific ways in which a person's APP differs from whatever universal standards of beauty may exist. Someone with a Roman nose might be considered especially handsome or lovely to a Roman, while someone with a dusky complexion might be considered more attractive to a Zazamanc or Spaniard, etc.

I'm not sure there's much to be earned from going into this level of detail. For one, you're going to have to work out how to translate between standards of beauty, which will either be simple and no more precise than before (-3 APP if dealing with another culture) or detailed and too fiddly to use in play (A different modifier for each culture pairing, not necesarilly the same both ways!)

As Undead Trout mentions, distinctive features already let you mark why someone has a high or low APP score. Circumstance modifiers keyed to those (say, dusky skin being seen as more a sign of being common born to northerners but more acceptable to spaniards) might be better handled on a case by case basis than with a hard-coded rules-set.

Pendragon characters aren't really random samplings of their people but are almost always the nobility - certainly in 5th edition. I'd argue that the nobility of western europe have quite a lot in common with each other no matter where they come from and that it isn't too ridiculous to say that the ruling classes probably had ideas of "beauty" that were reltively compatible. It's also not something that ever seems to come up in the fiction - certainly, at no point in Le Morte D'Arthur does anyone suggest that someone "Would be beautiful... to a Pict, I suppose."

KAP is a game about the matter of Britain and Arthurian legend. The cultural standards of beauty should be the same of the medieval standards of beauty and I think the rule about APP is correct. Because in Arthurian's sources and in medieval times, the standard of beauty was the fair lady with blond hair and blue eyes, white skin and so on. So the Danish women and men are APP+3 and Picts are APP-3.

It's like the "medieval" standard of warrior, it is the knight ! And more, the Christian knight (it was the reason why the legend of Saladin presents his secret conversions). Any KAP's players know it is more easier to have chivalric bonus with christian religion !

It's not a problem in my opinion. It is essential to understand the matter of Britain and maybe to play KAP.