Alright so this is going to be a long post but after playing the match-up alot during v1.1.0 then getting in a xton of games in on v1.1.1 I think I can finally weigh in on this MU.
Honestly
@ChileZeRo and MVD make this MU look even but Zero still says its at a disadvantage for us. Honestly ever since the addition of Dream Land I have felt that we had an easier chance dealing with this MU since Shiek players can't get rid of Town & City and Halberd.
In the neutral - At first I felt since the loss of our Uair as an approach/spacing tool we lost the neutral but its actually not bad it feels very even as I feel when comes to Shiek vs Diddy in the neutral its more based off on who has the better fundamentals and whos better at carrying the momentum. Momentum is the key word in this MU especially in this state as I feel whoever is better at controlling the neutral in this MU can basically win the match.
On the offensive - Diddy does very well against Shiek once he gets a hit confirm especially since shes a Fastfaller and we even more reward for our buck since shes light. If your combo/pressure game is on point it should be no surprise to get Shiek from 0-50% of of one read/setup/punish.
With the Banana in hand we are even more lethal as every character in general has to respect Diddy with the banana as one whiffed or over commited attack can be a free trip into a smash or a combo for big damage so I advise all Diddy players to really work on controlling their banana game as it really benefits highly in this MU learn to take alot of advantage off of getting the trip as not many characters are privaledge to have that in this game.
I mainly mix up With SHFF'led Fair/Bair with Dtilt for pressure and to feel out the Shiek. I feel Fair should be used more due to its range but it can be very predictable using it over and over which is why you should mix it up with Bairs. Bair is a really great aerial Diddy can do it twice in one short hop and can also mix it up to keep the Shiek honest example: (SH Bair twice to force them to shield then SH Bair once into monkey flip for the grab)
Luckily due to the new shield mechanics Diddy's pressure game is more reliable Dtilts and Fair are more safe on block...as long as you spacing is on point of course.
On the defensive - As I said earlier this MU is heavily momentum based and Shiek can turn the tides in her favor I feel Diddy has a higher damage output but Shieks attacks tend to put characters in great situations for her to frame trap with 50/50 setups and Diddy is no exception a well spaced Fair with Shiek is safe of Shield (unless you have a banana of course) and should be respected if you try to carelessly run in she even cover herself with a buffered jab/dtilt to stuff your approach. Both characters have an amazing grab game so Shiek's should be respected as well as they put you in a very tough position for her to continue a juggle/combo/or get the kill so learn to mix it up when in those situations and dont do something silly like airdodging carelessly.
Edgeguarding Shiek - Obviously Diddy doesn't do well offstage (hes not bad off stage just not amazing as Shiek,Greninja,etc.) again as I said earlier in the post having a great banana game REALLY helps your pressure game not just in this MU of course but in general when Shiek is usually off the stage I try to setup something before she grabs the ledge by either throwing the banana up and Dtilt the ledge, setting it behind me to cover the roll get up,keep it at the ledge in case they do a regular ledge getup, or Shield with it in hand anticipating a ledge release attack. Ledge trumping is also an ideal option to use I have gotten plenty of kills off of Ledgetrump Bair/Dsmash.
When Diddy is off stage - This is tough...VERY tough I mean how can I start? Shiek has Needles,Fair,Bair,Nair,and of course the infamous Bouncing Fish to deal with you off stage. To tell you the truth I rarely recover low actually...I do my very best not to even Up B with Diddy because once you do it especially against a potent Shiek its usually a stock lost.
When Shiek is edgeguarding you - So you made it to the ledge and everything is all good right? Nah that was just the beginning lol Shiek's edgeguarding setups are pretty insane especially if she has fully charged needles. She can do nice setups like Burst Grenade on the ledge to force you to get out of the situation and then punish depending on your option,dtilt you off the ledge, cover your roll with pivot ftilt,catch a ledge jump with Fair...many options just like Diddy. There are ways to get out of course just stay calm and dont over commit and do your best to turn the tides around.
When Shiek has needles - Yeah this deserves its own section as I feel its a BIG deal in this MU. Have you felt like you had space and decided to pull out a banana only when you land you take a whole bunch of needle to the face? Yeah...that can be a pain and good Shiek's will look out for that as the banana is a threat to them one thing I learned from watching ZeRo's Diddy is that sometimes he'll run away do a SH Banana Toss then quickly land and shield. Usually in this situation the Shiek will either throw the needles or pursue you and then throw them once you pick the banana up. What I noticed as a safe option against Shiek is to walk forward and shielding while staying behind the banana. This helps you stay alert to needles without creating an opening for the Shiek player to punish. If she throws the needles you can quickly shield then pick up the banana and handle the situation. Do they do a well spaced Fair instead? Good immediately Fair or Bair out of shield. Did they pick up the banana while Fairing you shield? Even better as they wont be able to retaliate with the banana in their hand until they land to the ground which of course you wont let them do.
Here are some videos of Diddy's winning and losing in the MU to get a visual experience of what to do I feel even the 1.1.0 vids are still helpful as things have changed but they aren't major.
Matches showing a great example of Diddy in control
Matches showing a great example of Shiek in control
Sorry for the long post. It would be nice to also get an input from Shiek players like
@ArikadoX. Any input is appreciated.