If you want to make your bike look undesirable, I'm pretty sure you could use "rust paint" (ie, paint that is designed to look like rust). Although, how to most effectively apply it so it doesn't scream "fake!" is another question. Perhaps taping off small, odd shaped areas and painting them this way and spray-painting chrome bits like this might be one way... maybe. But again, my main concern with this is that it would look really fake... so it would probably require a degree of artistry/trial and error to get right.

Also, if you've got a nice seat, you could probably find a crappy seat, remove and thin out the foam, cover your nice seat in 1/8" or less of the foam padding, put some rips into the pleather seat cover from the crappy seat, and use it to cover your seat so that you have a really comfy seat that looks like a piece of crap.