He was a fantastic heel, I definitely disagree with you there. He certainly got more heat than he gets cheers as a face. Fuck, I love Morrison, but lets be honest here, nobody gave a flying fuck about him until the Sheamus and Miz feuds at the end of last year. He was about as over as Chris Masters. He's not a bad face, but he'd be a much better heel.
No, he's really not. His ECW promos were bad. They're better than most of his "flying tree" dross but really that's not saying much.
And this whole "Nobody who does flips can be a heel!" myth is so ridiculous. AJ Styles did it to perfection years ago, you just make the guy an extremely cocky and arrogant heel who wants to prove he's the best, so high-risk moves come with the territory.
The TNA crowd is completely different to a WWE crowd. Not just because of the size differences, but because the TNA crowd is the same one every week. We've all heard the complaints about the iMPACT Zone fans and the Cancer Crew. And while a guy CAN get flips booed, (Gabriel was and is doing it with his 450 splash) it's a hell of a lot harder than it is with a more grounded moveset, which Morrison lacks.
And look at how heels generally act in a match. They slow down the pace and stop the face from doing exciting moves. The exact opposite of what Morrison's moveset does.
Have him celebrate like crazy after a high-risk move like he just cured cancer and the crowd will boo him. It's not rocket science, moveset is truly irrelevant to face or heel alignment.
Then why have the only "high fliers" in WWE that were heels (that I can remember at least) were also insanely charismatic? I.e. HBK and Eddie Gurrerro. It's not an opinion that the crowd pops for high spots. Look at how the Hardyz (Jeff, mostly) got over, how Sin Cara's getting over, how Rey Mysterio got over, how Justin Gabriel got over on NXT and how Evan Bourne got over.
High fliers can be over as heels, but you're fighting against their fast paced, exciting moveset.