To me, it is an insulting term, such as that Megyn Kelly is a stupid blonde. That is how I took Trump’s remark. And I don’t think there is a way to overlook his calling Fiona ugly or saying that Carson resembled a child molester.
The main American definition of "bimbo" seems to be a woman who puts forward sexuality instead of brains. (It's originally an Italian word, meaning "babe.") Clearly, Megyn does have some brains.
But to be fair, Megyn has posed for soft porn photos that are on the Internet; she often wears very revealing clothes at DC dressy events; she went on the Howard Stern show and talked about her breast size, her husband's penis size and their sex life; and at that first debate where she and Donald got into a dust up, she wore a cocktail dress, extremely false eyelashes, lots of makeup and "bed-head" tousled hair extensions. Since then, she has been criticized for looking trampy and has toned down her look severely. So there might have been some justification for being called a bimbo, because she had been walking that line.
I didn't appreciate Trump dissing Carly Fiorina; that was unchivalrous and rude. But the Rosie O'Donnell feud is a different matter entirely. She had been badmouthing Trump for some time on The View and made it personal towards him and his family. So he let her have it, and they both got a lot of ratings out of it. But truth be told, when she had her heart attack, Donald sent her a get-well message.
New York tough guy talk is not well appreciated outside of the NY-Philly-Jersey area; but often such people do have a lot of heart. The verbal stuff is a sort of street theater. It helps if you live in a dense environment full of dangers and threats. I had a pretty terrible trashmouth when I lived in a city, to protect my family members in the neighborhood streets that were often infiltrated with bad guys looking to steal, rob, litter, assault or vandalize; and I'm a woman. It just went with the territory.