I guess I'm in the middle on this "does TBBT get nerd culture right?" question. It depends on what you're talking about. The writers frequently don't seem to get it on aspects of SF fandom. If that's what you mean by nerd culture, then yeah, I agree that it's often written from the point of view of a mainstream person's outside view of nerd culture.
But if you're talking about the portrayal of physics academia, then it's often been scarily spot on, at least in the early seasons. As someone in astronomy/physics academia, I thought some of the discussions in Season 1 about teleportation or jokes about string theory and the like were great. As late as Season 3, there was still some good stuff in that regard, though it was less frequent. Raj and Sheldon working on dark matter research by staring at the white board for hours was hilarious.
But in the more recent seasons, their jobs at the university are just not much of a focus anymore. The show's much more fixated on the relationships between the characters first and foremost, with their geeky SF fandom second, and their jobs as physicists a distant third.
EDIT: Should also add: Everyone I know under 40 years of age within physics or astronomy academia who has sampled the show in its early seasons enjoyed it, at least a little. Never met an actual physicist who found it insulting.
I pretty much agree with all that you've said. As I mentioned earlier, I feel that the "science nerd" aspect is pretty accurate. The "scifi nerd" references used to be more obscure but have always tended to be those more understood by the mainstream. This doesn't bother me so much though, because why should anyone choose to make a show where 85% of the audience has no clue what the characters are referencing or why it's supposed be funny? Sure, it would be hilarious to those of us engaged regularly with that culture, but to direct the show only towards us would be pretty limiting.
Anyway, I also agree that the science jokes, while accurate, have died down in the past few seasons to focus on the relationship dramas, and I do wish they would go back to that sort of humor more often.