You should not sticker people's cars. It would be much better to put it under their windshield wiper like an advertising flyer. This will give them the opportunity to consider how their behavior impacts others. If you're trying to educate someone, your first step should not be doing something that enrages them. Furthermore, you don't suddenly gain the right to vandalize people's property because they are acting insensitive to the needs of others or in violation of the law.
As another commenter posted out above, some folk just aren't receptive to polite requests not to be an asshole. In these cases, I fully endorse public shaming.
Who said it would work? I just think unrepentant assholes deserve to be publicly shamed.
I don't think we're dealing with unrepentant assholes here. We're dealing with inconsiderate idiots. "Never attribute to malice that which is adequately explained by stupidity." I know I've hurt people out of stupidity my share of times. St. Bernard of Clairvaux once said the the cardinal virtues are "humility, humility, and humility." When we see bad things in the world, we should try to correct them, but we shouldn't automatically assume that the people doing the bad things are always bad people. That is, unless we're all bad people too?
Until one knows the reason someone may have parked that way, one should not take action. This is what's wrong with every strand of vigilantism. Sure, it could be simple inconsideration, but that usually isn't the case; some other believable causes for the behavior could be simple ignorance (which calls for education, not vandalism), or emergency circumstance (which makes stickering the only antisocial behavior in this case), just for examples. There is no good reason to forgo the benefit of the doubt. On an emotional level, OP isn't acting in a "mature" way, or being brave, or a force for good, or anything of the like. This behavior is cowardly, passive-aggressive, and slightly criminal, and sooner or later will likely end in what I can only see as a much-deserved beat-down.