In general, I tend to think that slip-ons don't look as good with a kilt as lace-ups and buckle shoes do, but a lot depends on the overall effect. I think of slip-ons as a casual shoe, so the rest of the ensemble needs to match. Dress pumps look best with tuxes.

That said, I also tend to think that a well developed personal style worn deliberately is almost always the best accessory.

Regards,
Rex.