Due to the glut of content available today, viewership is fractured. It doesn't take much to be the "most watched TV show" any more, just like it doesn't actually take many sales to top the music charts. Something can hold a record for viewership while still being something that most of the population (or most of a subculture like us here) remain oblivious to.
Also, HN is a global community, and even though many television shows are watched internationally, their impact on pop culture overall can be drastically different. So, you can't assume a major show in your country will be readily recognized by your fellow nerds in another country.
Even if that is true wouldn't you agree that NCIS is mainstream? If it's not mainstream I don't know what is. Feel free to give a better example.
Let me remind you that the whole discussion started by me highlighting the popularity of The Last Airbender (due to Netflix, as someone pointed out) by comparing it to a mainstream show. I could have taken any popular show but thought taking the one topping the charts would be enough to illustrate it. Apparently not, and in the future I must carefully review the viewing habits of the whole HN community in order to make a point and not offend anyone.
So let me revise the comment: The Last Airbender is as mainstream as any other popular show today.
Also, HN is a global community, and even though many television shows are watched internationally, their impact on pop culture overall can be drastically different. So, you can't assume a major show in your country will be readily recognized by your fellow nerds in another country.