The problem with Sheldon, and perhaps any stereotype in any show is, that it's a scripted, over the top stereotype and viewers will not see this as such. They see it as a character, not as an over the top representation of something, just like people rarely watch an action movie and state "that's impossible".
It's entertainment. And therein lies the flaw, presumably, a lot of people miss. Sheldon is only as quirky as the writers tell him to be. However because people don't see this as "he's just written, acted and portrayed this way" people actually perceive him as a real person, even more so than say... Wolverine or Optimus prime. Those last 2 border too much on "the impossible" while Sheldon is the ?ber geek, which, while he might be a big exaggerated is something more in place with the real world.
If you add a parallel and popular notion that geeks and nerds are often perceived or linked to Aspergers (and specifically Aspergers, not Autism. I have spoken to people who used the word Asperger's and weren't even aware it was a form of autism) it's easy to think Sheldon fits this condition perfectly. Unless... you're an aspie. You might still think his jokes are funny, but Sheldon on the spectrum sounds more far-fetched because you're more aware of what it is to be on the spectrum. It's the kind of awareness aspies have when it comes to the portrayal of (presumed) other aspies. It's also the kind of awareness non-aspies might not have.
What's also worrying about this all is that because BBT is a popular show, people will watch this show, assume something and it becomes a popular opinion. People think Sheldon is quirky, someone on reddit (or facebook, or tumblr, or whatever social media outlet) pitches how Sheldon shows severe traits for Aspergers Syndrome and the cat is out of the bag for the masses. Sheldon is an aspie, even though the writers never said he was. It's what people think someone is and it becomes a popular opinion (and by now I know all too well, a popular opinion is not always the correct one). It's these kind of opinions that feed the ignorant. It's on a forum like Aspies Central, where we (the aspies) actually question the validity of him being a potential candidate on the spectrum. I'm going out on a limb here, but I don't think this is the type of conversation families have over dinner, even if they might all gather on thursday evening to view the latest exploits of Sheldon and company.
So does the BBT add awareness for autism and Aspergers?
It gives a faux awareness. It gives an awareness of what laymen (and women) think what Aspergers is. It's the same kind of awareness Rainman did for Autism. And Raymond Babbitt, as a character was written based on Kim Peek, someone with FG syndrome and not autism. Yet, because autism seems to be a more popular label to throw around people assume that this is the correct label even if it heavily exaggerates a stereotype, who operates in a world that accomodates him as much as scriptwriters allow it.
Sheldon as smart as he is, would have a presumably harder time in "the real world" since I don't know if people would keep up with his antics. I once jokingly told my girlfriend "If Sheldon was he get beaten up on a pretty regular basis. His antics are probably too much for most.". This in turn reflects how even that tough guy ex-boyfriend of Penny is written off as a nice guy and not as the overly violent "shoot first, ask questions later" type of person. Afterall, it's still a sitcom. When was the last time a sitcom got violent and graphic? It would lose viewers since it would do a full 180. Not to mention that CBS wouldn't stand for it since they most likely want reliable content that will not alienated viewers. And this actually adds to the tone of any tv show; they don't want to blindside the audience.