As someone who mains Ness and has plenty of experience with Sheik, here's my take (might be a tad bit short due to it being late, will add more later):
Sheik has potential to take advantage of our floaty selves, being able to get all of her combos even at high percents. She's light in weight, which makes it easy for us to kill her, but no so easy to rack up the damage. However, Sheik is likely to develop habits quicker than the Ness player in this matchup due to her having a smaller set of things that actually work most of the time. What I mean by this is that a decent amount of her options aren't good to use, while we have to experiment in order to approach and whatnot.
Sheik's grenade isn't that great against Ness, easy to get heals off of since it pulls you in, not to mention they'll be in stun, meaning it'll be harder for the Sheik to punish. Bouncing Fish isn't that great against Ness either, because he's short and has a high double jump, allowing him to just remove himself from a situation that he expects it, not to mention pivot PK Fire. Probably the biggest thing that Sheik has trouble with against Ness is actually killing him. Ness has some very good hitboxes with FAir and NAir, allowing him to have a quick escape before being killed by the majority of her kill moves. The main kill move to look out for is Up Smash, because we're floaty, she can capitalize on that.
For Ness:
Experiment Approaches, what's worked for me were DJ Cancelled PK Fires to set up a wall-like box that Sheik is stopped in, useful when you see they are just approaching with Grabs a lot, since it baits positioning but then catches them in a trap.
Mash Jump + Away out of any of her throw combos, then try to land with a falling NAir.
Don't put yourself in Up Smash range, stay away and mix up your directions when falling, keep in mind that our PK Magnet stops our momentum for a bit, which means you can use it to bait an Up Smash from her and procede to punish (I have successfully done this several times).
They'll be wanting those combos to rack up damage, because as said, they need every bit of % they can get in order to kill you.
Mix up Pivots with the Neutral stance, I wouldn't recommend approaching when they are, since it can accidentally set you into Up Smash's sweet spot.
Back throw kills very nicely, but make sure to pummel a lot to be certain of the kill!
Ness v. Sheik feels like an even matchup from my perspective, however, I will say that I MAY have a bias because I know how to use Ness and know what he can do, and the inverse with Sheik. I admit that maining both may make it a bit bias'd, but that's up to you to trust or not ^^