Standing up for free speech is a great thing to do; to reframe what you are saying we are dealing with one of the mildest outlets for anger that a demographic could possibly express. We want people to do that rather than escalating to, eg, yelling at people in person.
People get angry all the time and a lot of them don't know how to deal with it. A political cartoon may not be a shining example of debate but it is a great way to let people see that their feelings are understood and broadly communicated.
If it doesn't cover expressing an unpopular, offensive and unimportant opinion as an image, what does free speech mean to you? Are we only to have free speech on topics are declared important by popular survey?
Just to make it clear why that is a ridiculous idea, the principle of free speech is ultimately not for majorities, it is for minorities. You don't really need a concept of free speech to talk about things that are widely recognised as important and worthwhile by everyone. It is top protect things that are generally believed to be better left unsaid. Things like purposefully upsetting people with a cartoon.