Of all the political memes I've seen here over the last 2 years, I'd estimate that 90%, at least, were nothing but vindictive drivel. While advertisers may have agreed with some of it, they also agree that it doesn't help if their target audience is pissed off by the content of the site. Why do you think most sites no longer allow nudity or more explicit images. Every featured post represents not only the poster, but ImgFlip's corporate image. If advertisers feel that a large percentage of site users are upset by what's featured, they will also choose to boycott the advertiser.
Imgflip has done something most sites don't: they try to have responsible advertisers. It's taken them 2 years to try and rein it in, but in the meantime, how many good meme makers have left because the political meme makers were so disruptive of the fun aspect. Many of them chose to cyber-bully others. Some used bots, others used a circle of friends, especially when pressed for facts.