Some useful things for Ness to know:
If you hit and catch her with a PK Fire, don't approach sideways, as she can escape with Nayru's Love (which also turns the fire to her favor). However, Ness' short-hopped Dair will hit right through Nayru's (which has a blindspot at its top) and will also keep you safe from the 'reflected' PK Fire.
Dtilt has a high trip-rate and locks you pretty well, but you can always attempt an SDI (away from Zelda) to get out. Shielding while doing so might help as well. ALWAYS DI up after you're about to get hit by a follow-up move (which are mostly Dsmashes, Fsmashes, Dash Grabs or Dash Attacks). As for grab-release to Dtilt, try to DI away from her after the release and spotdodge. She can re-grab after a Dtilt trip which can lead into a Dtilt-inclusive semi-chain grab.
Your properly-spaced Fairs (a spaced Jab CAN beat it) out-prioritize most of her moves and make a useful approach and set-up starter. It will even beat high-priority moves like Nayru's Love (from behind even), penetrating right through the invincibility frames. In my opinion, knowing how to space this without getting punished is essential in this match-up.
Don't get fooled by Din's Fire. She can always still use it as a pressuring option, or to bait openings and absorbing-attempts. She can always predict a PSI-magnet attempt and cancel the Din's Fire. Offstage, occasionally letting yourself get hit by Din's Fire (WITH proper DI upward) might help you gain elevation and prevent possible gimps. Nonetheless she can pressure you extremely well with it offstage, so avoid being next or below the ledge (while far away from it). A mildly charged Din's can also break your PK Thunder (lol), and optionally she can jump in and reflect with Nayru's Love (which is risky so you won't have to expect it often).
Do NOT try to punish her perfectly spaced Fsmashes with anything but a PKFire or Fair OoS (which will at least stab her shield and sweep any possible openings). You'll most likely get punished if you try anything else. Her Ftilt actually hurts grounded approaches and can catch you out of some aerial approaches in this match-up since most of your attacks won't reach (and punish) the hurtbox on the move.
That's my two cents for this match-up. I might add some more stuff to it later, but this'll do for now. Also,
Eagleye893 said:
Don't try coming down with psi magnet unless you know what you're doing... might get uair'd, usmash'd.... anything will be bad.
This. I've seen so many Nesses Magnet Stall high up in midair whenever I approach them from below, and they always seem to get punished for it (lol Uair). High recoveries for Ness are also pretty dangerous, unless you're really fast lol.