I'm pretty sure everyone is blowing this Diddy thing out of proportion. While he probably didn't need any more nerfs, I can see what Sakurai is doing. He's watching everyone play Diddy the "brain dead" way, because of how effective and easy it is, and thinking he's done something wrong in balancing his moves. It shows in his balancing decisions with his play-testers.
People mindlessly spammed Nana -> grab combos and Monkey Flip and then Uair and Fair spammed to victory. When that failed, they threw out the rather rewarding F-Smash. No one ever jabbed, barely any tilts or other smashes were used (minus maybe d-tilt) , DA was obselete, Popgun was unnecessary and saw little use at all since MF and Nana approaches were fine and the other aerials may as well have been filler, minus maybe bair once in a while.
Diddy was unfun to watch, and didn't even look balanced. Half of his arsenal was braindead and the other half was obselete, less useful or plain useless. This is clearly not how Sakurai defines a fun character. Its been pretty clear that when certain moves see no mileage due to flaws and others see too much mileage because they're so good, he rebalances them in a way that he feels puts them on par. MK's jab and f-tilt were weak, so he buffs them. Greninja's Up-Smash was ridiculous but his f-tilt was crap, so he adjusted them. Ike had a terrible DA and laggy aerials outside of fair and bair, so he improved them. Diddy is not exempt from this philosophy.
So what have these patches done? Nerf the biggest offenders (Uair, fair, MF, Nana, F-Smash, U-throw/D-throw) and then buff his most useless attacks (jab and U-smash, which links better). Sakurai clearly wants the characters to be played with all of their tools, and not just a handful of broken ones. He wants every move to be useful and wants everyone to consider all of their options instead of "l0l nana toss, garb, uairuairuairuairfsmash lololo0lol0lol." Its kinda obvious that the mindless uair spam and generally braindead Diddy play was what put the nail in his coffin. I feel like he would've been hit less hard if people would have been a bit more diverse in their use of Diddy. But hoo-hah sealed the deal. We probably have only ourselves to blame for his neutering. And Sakurai to blame for making him so good in the first place so that he could be played this way, and then taking the wind out of his sails after so many became accustomed to him.
That said, he's still fast, has great tools, rather lagless attacks, a versatile projectile game and fluid play. He just gets punished harder now for rushdown, making him like MK; risk and reward for getting in close. Only replace low reach and a light weight with a bit of ending lag. He's still solid everyone. Don't act like he's Brawl Falcon or Dorf.
As for Sheik, her terrible damage output, even worse killing tools and the fact that most of her moves actually see use likely makes Sakurai think he's done enough to her. She's already been nerfed three times. Yeah she's still stupid good, but she's diverse and takes a high understanding to use at top level. Diddy...he kinda didn't. But now he probably will. @
TSM ZeRo
will still excel with him, and he'll still perform well. It just won't be as easy.