It's not so much the specifics. It's just him being ignorant and rude. Doesn't matter if it's a joke.
Again, if i made fun of some kid with downs syndrome but then prefaced that with "oh i was only joking!" what would that make me in your eyes?
Hopefully you'd think i'm a raging *****e bag.
What did he say that was comparable to making fun of someone with downs syndrome?
On shehulk,
If someone made a joke like say, the various theories about superman having sex with lois in that mall rats movie or that other well known superman wonderwoman one. Then saying you have the right to treat this same person in the way you would someone that was making fun of those afflicted with downs syndrome is....opportunistic imo. Just another chance to point fingers at someone you clearly have something against. Like when obama says something silly and fox news goes nuts.
Unless of course this is about him poking fun at a social group he himself admits to being a part of. Making fun of basement dwelling beard necks and all that is hardly a crime. Again, it was a self deprecating joke. It's not like he was a jock being asked about the character and just answered with, "ask the losers". It was a simple parable and commentary on the characters lack of notoriety.
Even if he wasn't joking it wouldn't be that big a deal. This isn't downs syndrome, so throwing that term around..never mind.
Secondly, I don't even get what he said about J'onn. That the full name is unnecessary and lame. It's not like all that many superheroes these days have the term 'martian' in their name. It's not like 'hawkeye' or some other code name. The character has various other alias' to go by.
Does anyone even know the story behind the name?
If this is about Goyer thinking the MM character wastes his powers. So what, many people feel the same way about many characters with god like powers. That doesn't mean he's going to write the characterization all that differently. It's not like thinking Bruce Wayne wastes all his money on crime fighting is going to make me write a batman that spends all his time in the play boy mansion...oops better be careful, that joke was maybe in bad taste.
I personally like his angle with the dna. Reminds me of the 5th element. Especially if he then used as a gov't tool(at first).