Technically, no. There are tons of rules and regulations about signs. A lot of it is under zoning. I'm not a lawyer or anything. I know about this topic from Christian ministry (starting a storefront mission, thrift store, etc). Here's a good real life example, I think. A code enforcement guy from the city says that my sign is illegal. Then he says I need a hundred thousand $ insurance on the sign. Then he tells me the general parameters of how he'll likely ignore the sign. Basically if it's not too much of a nuisance then the authorities ignore it.

Ask yourself- How many people do you know who have been fined for stapling a paper flyer to a pole? For me it's zero. "Everybody does it." Right?

It's just a meme? Well if it's political or otherwise causes outrage then it may be seen as a nuisance, so the authorities might get involved. I suppose it depends on how hard you kick the hornets nest, and blind luck.