We generally hear how cute Lao girls really are from Vientiane and up north to Vang Vieng and Luang Prabang, but southern Lao girls from Pakse and Champassak are not really known for their beauty.
This is because Lao girls from the southern region of Laos mostly have dark skin, with the exception of a minority with white fair skin.
The climate in southern Laos is always hotter than the upper regions of Laos, and it contributes to darker skin colour. Also, most southern Lao people are naturally dark skinned and there's not much they can do to change this. There are many mixed Lao-Vietnamese girls in Savannakhet and surrounding areas, so not surprisingly they tend to be white or lightly tanned.
Like other Asian countries, pale white skin represent youth and beauty in Laos while dark and tan skin is a sign of manual labour and poverty. The "white Lao girl" look from the central and northern regions of Laos are highly sought after because they are first and foremost lighter than the average Lao girl from southern Laos.
Lao girls from Vientiane and Luang Prabang are also more modern, they have more income and more likely to have completed university/college than a majority of Lao girls who live in the south.
Some men seek southern Lao girls from Pakse and Champassak because they are known for their natural looking and faithful characteristics, and they always follow religious and traditional Lao values. And there are men who have never really been attracted to southern Lao girls due to their physical features, I guess we all have our own cup of tea.