I'm gonna take a legit shot at this.
First of all, the DDT can work very well as it is obviously very dangerous. If you land it and it doesn't stun them or knock them out, they're past your guard, and that's a big problem in MMA. Especially because they won't be too happy you just fucking DDT'd them.
As far as the legality, it probably IS legal in the unified MMA rules. Their definition of spiking involved "inverting the opponent before dropping them on the top of their head", which basically means.... piledriver. Even within the past 2 years, there have been two or three actual piledrivers in the UFC that have caused serious stunning or directly led to the end of the fight and the refs never called it. Even a great ref like Herb Dean didn't call Nate Marquardt for piledriving a guy, so a DDT will probably fly.
Also, there are different DDTs. You can plant them on their face, forehead, or top of the head. The first version would probably fly while the second version would be more likely to get you DQ'd. (This is all 100% guesswork BTW)