Rapid jab isn't very good unless on the edge, if you just hold A you won't go in to it.
I can't say for certain at the moment, but I recall reading that most, if not all jab -> grab combos are either impossible or situational.
This is what happens when a new game comes out. People quickly jump to conclusions instead of figuring out how to deal with certain things. When Brawl came out, Dedede was 3rd, GAW was 4th and ROB was 6th on the community tier list. These are characters that had gimmicks that nobody wanted to learn to deal with and thus inflated their performances. Once people learned the game, they began to drop repeatedly.
Diddy isn't unstoppable, just like in Brawl. Stay in his face, bait the banana pull, attack him before he grabs it and steal it and use it properly. This kills the Diddy.
If there are any more balance changes to come, Diddy will probably see attention to cater to the crowds of people that make the issue seem larger than it is. Rosalina was another instance of people not bothering to learn her. Many characters had options to go through most of her moveset and her gimpability was only impacted by us all playing with inferior control options on a 3DS. She still would have been very good, but she wasn't broken.
I learned from my three+ year stint at tournaments that you should just play who you like, community target or not, and have fun doing it. Haters are going to hate, and they are free to pick up the character and attempt to prove that they're so broken. That's when it really gets fun! Anecdotally, my Diddy crushed KirinBlaze (One of the top Mario mains at the time) Game 1 at a Clash of Titans. He took me to Rainbow Cruise, one of MK's best stages, and he went MK. This should have been an automatic loss, but I still managed to 3 stock him, and it felt damn good. If a character is truly broken, anyone can just pick him up and demolish with basic understanding. MK required skill and knowledge, otherwise situations like Vermanubis beating MK's with Ganon would never have happened.
The thing about Diddy also is that he is one of the least changed characters from Brawl that transitioned well to Smash 4's slightly less defensive environment. If you knew the character from Brawl, you were going to get the hang of him fast while many other character mains are busy trying to figure out their combo setups and adapt to changes. You seem to have been really excited to play Rosalina, and while I'm always happy to see another Diddy main around here, you should go for it if she's still in your interest. Diddy will get worse as people learn and techniques to counter Rosalina will pop up if need be. Don't let others ruin your good time, main whatever feels most natural and try to have fun