It never presented itself as a totally serious universe even from the first game
maybe you don't remember the SC1 cinematics, the terran ones were always goofy.
Eh? How is it goofy? Funny because of the redneck accents, yes, but not goofy. The goofiest thing in there is "I love you Sarge," which quite frankly is an appropriate emotional response from someone who is about to get brutally eviscerated. It's pretty damn dark, and don't delude yourself into thinking that you or me would have said something "cool" or "badass" in their place.
I haven't watched the cinematic yet because I'm tired of having 2/3 of them spoiled before the game is even released, but I can see why people would get irritated at the contrasting tones. Personally I'm always way too amazed at the CGI to give that much of a damn, but this type of half-shod writing is present throughout all of SC2 i.e.
Dark Voice does something as epic as wiping out an entire race....then spouts a quote from A-team.
Tosh is brutally murdered...and then we get a stupid voodoo doll scene with Tychus. The writers said in an interview that they only put that in to "offset the darkness of the scene", almost as if they were afraid somebody would take them and their campaign too seriously.
The bottom line is that this story is made to be "accessible" to 9 year olds, and so we can't have any serious scenes without them getting brought back down to silly fun play land half the time.