This MU revolves purely around dodging MK's dash attack and grab.
While he cannot optimally follow up after a down throw after certain high percentages, mainly in the 90-100% with a bit of rage on him, he can still kill after a down throw at 50-70% if he baits the scare dodge and then up-b's you to the ceiling.
However, while playing against MK keep in mind that it is NEVER an EVEN game -- one dash attack can reset the score board after a correct follow-up string to kill.
MK also has to mix up his landings either with crossing up a dimensional cape or d-airing back to safety which are things you can start to get a read on as the opponent reveals their habits.
Your Advantages
- needles
- solid bouncing fish follow ups at normal percents, meaning he's not super floaty
- your overall better neutral game with access to soft nair, fair, and the typical b moves
MK's Advantages
- a very solid percentage range, 15-25% depending by which he can kill you by making use of Sheik's fast fall speed
- raw kill power in fsmash, up b, and dimensional cape
- returning to stage is free either through up b, side b, or down b
In short, if you don't get dash attacked, the game relies more on Sheik's setting of the pace.
Otherwise you will likely die to uair strings since they are not that hard to get and the DI mix ups is all you can really do besides them messing up their inputs.