I'm going to have to step to the side of the NEC/TTI marketing folks from back then on this-when you compare the ads for Johnny Turbo or even Air Zonk, they were in line with many of the ads that were released back in that era.
Game advertising in the early 90's was all about getting in your face, having attitude, being off-the-wall, anything to really separate one from another and rise up from the behemoth that was Nintendo. And many were probably as goofy as Johnny Turbo. Look at the original Sonic ads: you just saw a blue blur that spanned the bottoms of two or three magazine pages before you got a full page ad asking who hit pause. We were told he's fast and had attitude. And.....? What's so big about a blue HEDGEHOG? Why woudl I want to play as a blue hedgehog that ran fast? Of course, the proof was in the pudding when you sat down and played it or saw screen shots or actual commercials but in the early days prior to the game being released, it seemed a bit lame.
Then go back to the ads of Zero The Kamikaze Squirrel, Aero the Acrobat, Boogerman, Haunting (starring Polterguy), James Pond: Robocod (where he's seen in a soda shop asking Sonic if he can do the same tricks the fish could). Pretty much any "mascot" style ad trying to show off how great or off beat (and thus "cool") the character was ranked pretty close to that Air Zonk ad.
With that in mind, look at what the Johnny Turbo ads really are: a method of getting the Turbo name into a prospective customer's head. It wasn't a "last ditch effort"-it followed the trend of any other early-to-mid 90's game advertising. It was so ridiculous, whether you liked the ad or not, it stuck. I would bet some Meseta that Johnny caused some gamers to take pause when they walked past a Duo, snicker about Johnny Turbo and then actively look at the system to see what it was all about.
By looking at these threads, it's obvious the advertising was a success: people are STILL talking about the ads.
One of the big things, like others mentioned, that killed the Turbo in the States was not having recognizable games that could rope gamers in. By the time these ads came out, the Turbo's destiny was already carved in stone. All they could do was just go through the motions and follow the same advertising concept that everyone else did because it "worked".