Although it was probably doomed from the beginning, and they did some things wrong, I think TTi also did some things right when releasing the Duo too.
Generally the rebranding was good. Look at the packaging for the Turbo Duo vs. the TG16. The slick grey and black box of the Duo was a huge improvement over those two smiling 80's dudes on the TG16. The design of the system itself looked less like a toy than the TG16 did too. As motdelbourt pointed out, the system being able to play all available software released the confusion to the uninitiated consumer. Having built-in AV and back-up support was obviously good (though another controller port couldn't have hurt either).
The mascot change to Zonk was intended to be more 'badass' looking than Bonk, but I think overall this wasn't such a good idea. Both Sega and Nintendo had highly recognizable mascots by that point, so TTi switching at that point felt a little bit like a cop out. Zonk, a relative of Bonk, looked similar, but the similarity to Bonk made him seem all the more confusing to me. "Is Bonk still the mascot, or is this similar-looking character now replacing him?" As an early teenager at the time, I honestly didn't know what was going on. By changing the mascot, they wasted any small brand recognition they had managed to build up with Bonk up to that point. Bonk wasn't exactly a household name known even to parents like Mario and Sonic was, but he was pretty well recognized by gamers - maybe more so than the hardware was. His character was a unique and recognizable concept that I'd always thought Hudson, NEC, and TTi could have taken more advantage of.
I think I will play devil's advocate and say that giving away so many good games as pack-ins was a GOOD idea. What TTi needed to accomplish more than anything was to expand the user base for their hardware. That's why they included such an enticing array of quality games with the system - to get people to just buy it in the first place. They knew that if they couldn't do that, they were certainly even more doomed than they already were. And, by showing the customers what their new system was really capable of from the beginning they instilled an immediate love for the hardware for almost every one who bought one. Just look how many fans are still here on this board today. By contrast, sadly I'm willing to bet there were more than a few who bought a TG16 with only the pack-in game, thought Keith Courage was lame, and neglected or sold the system to focus on Sega or Nintendo's products instead. Personally, my first experience with the Turbo was renting a TG16 with Keith Courage and Blazing Lasers in 1992(?). I thought both games were pretty ho-hum. I already had an SNES at the time, and was unimpressed by the graphics and sound. I love Blazing Lasers now (and Keith Courage is not bad either), but at the time I was uninitiated into the world of shmups, and wasn't exactly blown away. Anyway, I am fortunate that I still decided to give the system another chance soon after.
And, at least not EVERY killer app was included with the Duo. If the Duo customer liked GoT and Bonk, LoT and Bonk 3 were just around the corner (though in quantities much too low in the case of Bonk 3, of course). Obviously, most Turbo fans can list plenty of other non-pack-in titles that would make their list of killer apps too. TTi was also probably hoping that establishing a user base would also lead to plenty more killer apps being produced for the system in the future. Keep in mind that they still didn't know at the time how many more potential killer apps were in the cards for the system thereafter.
