Do stickers or paint compromise a helmet?
I agree with the others, it doesn't seem like stickers should be a problem.  My helmet has stickers on the shell added by the manufacturer - clearly they were added make me ride faster.

Paint is a different story...  Polystyrene (aka Styrofoam) and certain other plastics can be damaged by the solvents used in paints, like some spray paints.  I believe petroleum solvents in particular can cause polystyrene to melt.  

Polystyrene is sometimes used in crafts and model building, so a hobby store might stock some polystyrene safe paints or be able to recommend a brand of paint that'll be safe to use.