I think Papilio cresphontes definitely fits the bill where the larva are not as pretty as the adult. However, I find them twice as interesting! Sometimes it pays to look like s*** (observation 126257250 below).
Later instars still look like crap, but they start to look like prettier crap (see observation 126257089 for comparison).
I caught a few of them from my citrus tree a few summers ago, kept them for observation, and successfully raised them all to maturity before releasing them. The butterflies were beautiful but I honestly think I might prefer the caterpillars… the little constellations of dots on them and the way their colours change from instar to instar is really beautiful to me, when I was keeping them I spent a lot of time just sitting and looking at them.
Wow, too many to choose from. Lots of bizarre Lepiodptera examples. Longtime fave of mine is the caterpillar of Phyprosopus callitrichoides, the so-called Curved line owlwet moth. The larva looks like something from a Dr. Suess acid trip: