When one says something, there's always some more information to be gleaned from context. Like yeah, I agree that some people use the phrase "it's just a joke" to be a dipshit, but I'd only bring it up if there was someone is being a dipshit.
By bringing it up in that context, it's giving an implication that you are agreeing with them, and it gives you the bonus of being able to wriggle out and go "oh actually I'm not on their side" once you get criticised.
In this case, the original commenter called out a joke that could potentially lead to harm
How is this going to lead to harm?
And if you're going to answer something like "oh they're going to think we're weird kiddie diddlers", the only person who would look at a meme of someone saying that they want to look like an 18-ish anime girl (especially considering that OP may be around the same age range, for all you know) and conclude that trans people are weird kiddie diddlers ..... are the people who already hate trans people and will come up with other reasons no matter what we do.
It has an iso of Hannah Montana Linux on it