I'm guessing their dog happened to get cancer shortly before a human loved one did and we're seeing what happens when you mix deep depression with honest stupidity.
Naw, a bunch of stickers of bands, states visited, funny shit, or 4wd trails is cool. (Like ok maybe they have ADHD like me or something). A bunch of political bumper stickers is someone who probably needs to ignore the news for a while.
But custom applied lettering forming unhinged screeds is, more likely than not, a condemnation of the government's failure to provide readily accessible treatment for mental illness.
I will never understand the impulse to plaster your car with your life philosophy. Obviously this is another level, but even the stick figure families or whatever are kind of weird. Some of it is sensitive information that you're just shouting out into the world.
For example, there's this person I see around my town whose license plate is "2HOUSES". So you're advertising to potential thieves that you're rich enough to afford 2 houses and that you're definitely not currently at either of them. Incredibly stupid just for the vanity of it, but you're also and just making a target of yourself.
In all fairness to them, dogs do have themselves a type of transmissible cancer known as CTVT (or TVT), but last I knew it's not transmissible to humans, and it would be very difficult for it to become so.
Of all the diseases you could worry about getting from your dog, cancer is not one of them.