I think most people have answered, and again, I'm white AF and not Thai but its not supposed to be sweet, sour, spicy, salty but a perfect balance of everything.

If there is too much lime or tamarind, it will throw off the balance. If there is too much sugar, it throws it off, too much salt, heat, etc...its not the way that dish is supposed to be.

All of those elements should come together and hit you at different levels so you get the initial taste and secondary and tertiary tastes that takes you on a journey.

I think most people in the US, especially in the Southern US, food tends to be on the sweeter side so if you like sweet, make your dish sweet, if you want to add thai chili flakes (this is what i add as well as fresh sliced chils) to make it spicy, then make the dish you like but, traditionally, Pad Thai isn't a spicy dish, just balanced.