Let's consider such a dialog on a social network:
"Yesterday a guy sent me a photo of his unshaven legs!"
"Oh, legs is/are just a stupidity, it could've been much worse."
Can I reply like this and should it be is or are?
Well, to answer your first question - Yes, you can reply in whatever way you like!
To answer your second question - The whole phrase sounds awkward. I would personally use "is" to emphasize the "is nothing but", however, it would be best if you rephrased the sentence: