Some characters are easier to execute, but matchups go beyond what is low and high tier. You can outplay a worse player.
I guess the point is every match is it’s own beast. The tier the character is will only come into play at a very high level and even then it’s still gonna be in the court of how well the tools are used.
Sheik has zero problems with neutral. You can combo into down specials for kills and her offstage game is great. On stage kills with smash attacks you seem to pretty much have to bait.
MK is a different beast that I don’t know all that much about besides his specials better hit if you use them. MKs airgame seems good from the other side.