IMO, it's not a bad matchup. Maybe slight disadvantage, and according to MikeKirby, even *at best*. Sheik is way faster, sure. But bouncing fish is maybe the scariest thing if you're not (foolishly) chasing her around. Sheik has more options, and at a certain percentage window, the 50/50 is a true confirm.Kirby is not even with Sheik. This is a misconception that has been going on for far too long and needs to die.
Kirby having comparable frame data and the ability to copy needles does not do enough to bridge the gap between his and Sheik's mobility. She runs rings around him, camps him, outranges him and is better at killing. To copy needles he has to catch her, which is a massive feat in itself.
Is Kirby Sheik's worst low tier MU? Possibly. Does that mean it's in Kirby's favour? Absolutely not.
But even without needles, patience in that matchup goes a long way. Rushing down Sheik is a great way to get wrecked in general. Also, Uthrow Buff helps a lot with killing on stages with platforms. You really don't have to wait for a hard read or chance-based trip-confirm to finish her stock anymore.
If we still had better needles than Sheik, it'd be more likely to be even... But I digress.