It's even against the TOS Conduct Section:
"In any content that you create or modify while using this website, do not discriminate against any sect, commit or condone illegal acts, advertise, harass others, or otherwise make it less fun for other people. Imgflip is a place to be creative and have fun! While Imgflip highly values freedom of expression, please express your opinions in a way that is kind and respectful to all people of the world, or if stating your opinion does not follow these terms, please keep it to yourself or post it elsewhere. Memes, comments, or other images whose sole purpose is to attack a person or group of people in a condemning and non-humorous way are subject to deletion, and the user account that posted such content is also subject to deletion. In particular, please be careful when authoring content around sensitive subjects such as politics, religion, and race."