It is so weird. I can't fucking find the word retardness in the net dictionaries, does anybody know if the word retardness was allready created?

I don't think so. Retardedness sounds slightly more proper.

If you use the word retarded, retrdedness looks logic.

But of you use the word retard, retardness looks logic too.

If you use the word retarded, retrdedness looks logic.

But of you use the word retard, retardness looks logic too.

From a grammatical standpoint, neither of your sentences look logic.

To answer your question, I think retardedness is a more sensible word. From my experience, it seems that -ness is affixed to solely to adjectives. Retard is not an adjective form so it wouldn't make any sense to add the suffix to that. On the other hand, you could just forget all of that and use the word retardation.