。
What do you mean, weird? That's ridiculous!
Literal
Weird [quotative-だなんて] such stupid.
だなんて quotes and dismisses what someone else said — 'to say it's weird, that's...' The speaker is indignant at being called 変わってる. そんな馬鹿な is an exclamation of disbelief: 'such a stupid (thing to say)!' The sentence captures emotional pushback in compressed casual speech.