Read your own definition more carefully: "not deviating from a norm, rule, or principle." This is not just a statistical definition; it's not coincidence that each of those terms has a normative meaning:
Norms are things you should follow (or at least things people think you should follow): "a required standard; a level to be complied with or reached." Same with rules: "one of a set of explicit or understood regulations or principles governing conduct within a particular activity or sphere." Same with principles: "morally correct behavior and attitudes."
If the word "normal" were only a statistical statement, I would (maybe) agree with you. But it isn't -- in exactly the way disclosed by that definition -- and we all know it. When a person is told that she isn't "normal" most people feel the sting (though sometimes, hopefully, the sting is masked or outweighed by pride). It is a statistical word loaded with cultural baggage. (Or perhaps the other way around.)
"Cisgendered" is a funny word. I get it. I would feel goofy writing it, let alone speaking it out loud. But it seems to me that the right thing to do is to suck it up and feel goofy when the stakes are another person's dignity.