Drunken cooking-sperging: Bob, I cook a lot. At least twice a day. I have marinated many chickens in my time. I have never heard of anyone marinating chicken in Mountain Dew, or any soda really. I doubt it would be any good, most marinades have salt in order to boost the moisture in the meat. But it could be good, I haven't personally tried it.
But if the Mountain Dew was just a quick and easy shortcut like you claim, why marinate for three days??? If you have three days, you have the ten minutes it takes to prepare a salt and water brine.
And why marinate for three days any way? I doubt the flavour gain would be noticable after 36 hours or so.
And even if that marinade is the most amazing thing ever and a salt+water brine pales in comparison, LOOK AT THAT CHARR!! You obviously cooked the chicken on way too high a temperature, or left it in the oven too long.
So to summarize: Even if you Mountain Dew marinade was any good (I doubt it), you burned your chicken and I'm willing to bet it was dry as chalk.