No, it's not true. You don't get it lol. I despise Sheamus, I would boo the crap out of him if I went to a live event. Does that make him a heel to me? No, not at all.
IMO he gets less boo's than he used to because they've either stopped trying to cause a turn or because at the moment he's not in any high profile feud (despite always being in the ME). Against Rock he was heavily boo'd, and it was hard to boo Cena against Brock because of how good Brock was as a heel.
Also, I dno't think I need to spoon-feed you storylines to persuade you. You're either on board with it or not, but it's really not hard coming up with different methods of turning him. You have The Rock -- one of the all time greats -- back sort of in WWE, how can you not come up with something to turn him? Also, kids boo any character change. Big Show was a smiley babyface who was cheered by kids, now he's a heel and he's boo'd by kids. They're not the members of the crowd which WWE struggles to manipulate, it's the older generation. Also, I think Cena's so hated to the point that we wouldn't even cheer him if he was an entertaining heel. It's cool to hatwe Cena if you're an older male and I don't think that'll change, especially against IWC darling faces like Rock, Jericho etc.