There is no evidence that being made of marshmallows is damaging to one’s reputation, and thus this would not necessarily be classified as defamatory. The law requires proven damage. The details are tricky.
I'm aware. That's why I mentioned it. There's no remediation if there's no damages.
If I'm made of marshmallows I'm not human and not eligible for health insurance. If my insurer believes your claim which is a reckless disregard for the truth and I'm denied coverage, you're liable. If my wife leaves me because she didn't want to marry a marshmallow and causes massive legal damages and emotional harms you're liable for defamation.
It's like you're arguing you have the right to punch. Like, sure, swing your arms wherever. But your right to swing your arms ends at the tip of my nose.
You don't have a right to lie about me.
However, there are very specific exceptions. You are not allowed to lie under oath. You are not allowed to defraud. You are not allowed to defame. Those states are each defined separate and apart from “lying,” which is freely permissible.