Well, the difference between Mandarin and Deadpool is at least Iron Man 3 did good with what they did with Mandarin, just at the same time, this act is going to piss off anyone expecting to see something resembling the character. As I said, remove the Mandarin name, and pretty much all my complaints go wayside. It was very clearly an intended design decision, and a bold one, just arguably not a smart one.
Deadpool felt more like the directors doing whatever the hell they wanted with the character, only so they could create a SUPER OPPONENT!!! for Wolverine to fight in the most un-creative way ever, and slap "Deadpool" on there so they could get people to see the movie by saying "Hey guys, Deadpool's in the movie!" and technically be telling the truth.
You could argue Deadpool was the same mindset, except the difference is that Iron Man 3 showed competence everywhere else that this came off as a "neat idea that WILL have polarizing reactions"; X-men Origins was anything but consistently competent.