No, if anything the "Horn" is a symbolic of the fact that there are some people out there who intentionaly use Mario Kart 8 to bash the Wii U and its Gamepad as a whole, but unfortunately in an incredibly ignorant way. Ignorant in the sense that they won't acknoledge the features that are literaly displayed besides the obvious "HORN" symbol and/or are already confirmed, namely:
- buttons to switch to an overview of the map
- displayed items held by every character, which in itself is pretty important for everyone who tries to master Mario Kart (so that you know what items people in front and behind you are holding, thus allowing you to act/react accordingly)
- not yet confirmed, but very likely I think: probably used to draw your on decals, since MK8 will have car-customization, much like MK7 but with "...a little more choice".
+ the fact that you will be able to use it for OFF TV play, and potential co-op features (which they have yet to reveal, but at least 2 player separate screen gameplay should be a given).
The horn argument is just of the place, because Mario Kart doesn't have to use it for more than stated above. I - and probably all those "horn critics", too - doesn't even know in what other way the second screen or the Wii U Gamepad could be used differently than what it's already used for, especially for a fun racer like Mario Kart. What is missing? What do people want? What else, besides of OFF TV play, displaying important information on the screen, improved cart customization convenience and Co-Op separate screen muliplayer should there be, in order to supstantially add "more" to the overall experience?
EDIT: Same goes for Donkey Kong Country: Tropical Freeze. It's a game that just doesn't need the Wii U Gamepad and I'm happy Nintendo doesn't force it's functionality onto it. This actaully proves that Nintendo will only use it where they see it fit. That, however, doesn't mean the Gamepad and it's cababilies are useless. Plenty of other games will use it in ways that actaully enhance them, just not Tropical Freeze.