Mac has a pretty similar game plan for most characters. The only thing that changes on his end is dealing with projectile harassment.
Sheik needles are some of the easiest harassment a character can have. In addition to pressure on stage while Mac approaches, she also has the ability to gimp him with aerial needles.
Sheik also has ftilt and dtilt, which easily convert to combos/links/chains/whatever you call them that end with the opponent in the air, either above Sheik or off stage. Both of these are horrid places for Mac, as he is either recovering or trying to land. Both of those things have limited approaches as Mac.
Her tilts are also fantastic get off me options, so you better be spacing everything perfectly. Ftilts at max range, body blows, rapid dtilts and such on your end.
From Mac's perspective, she's a medium weight character. You hit her, do the occasional throw mix up, and hit her some more. You'll want to dash shield needles on approach, and really make sure to not get grabbed. Pretend she's Luigi, because it's just as scary (actually, kinda worse) for Mac.
Overall, I'd put this as a 7-3 or so in Sheik's favor. Mac plays his same game, but the projectiles make it a bit harder. Sheik's damage method is also Mac's worst nightmare. This is definitely his worst matchup, in my opinion. I'd play any other character first.