I realize that the default desktop tooltips featuring a 128px icon alongside text is meant to be an enhancement, and there's not doubt they look very nice...when they do look nice, which is pretty rarely.

It's yet uncommon for developers to bundle a 128px icon with their applications. This means that usually a 48px icon is being stretched to fill the space. The result is tacky, to say the least.

I also realize that the size of the icons in tooltips can be controlled within theme gtkrc files via setting something like {
XfdesktopIconView::tooltip-size = 48 }. But the default should be to turn something with such potential to be problematic ON, not OFF.

48px should be the default. Larger should be something set by themes if someone has done the required groundwork to make it consistently display well.