The MSM have been lying about things Trump says for over four years. Twitter lets Trump bypass the MSM filter and lies. As to his manner, slanderers don't deserve "proper decorum". Further, Reagan, Bush, and Bush tried going the "proper decorum" route and, frankly, that was an utter cascading failure. Being polite to Haters just dignifies their slander and encourages and enables them to be worse. Trump, I think, noticed that and is trying a different route. It hasn't lessened the slanders, but his strategy has drawn the Haters into displaying their full crazy.
I'd love for there to be "proper decorum" and all that, but the Dems and their MSM shills started abandoning that back in the 70s. It's just taken a few decades for there to be an R President willing to respond. That's part of what freaks out the Dems and their MSM shills. Trump isn't laying back and taking it the way a R President is supposed to.