If you play as Kirby, use utilt.
Abuse it. Ness's dair may be beast, but Kirby's utilt is even moreso.
Shielding Ness's dair is often not the best idea because it has so much shieldstun, leading either to a free grab for Ness or the breaking of your shield if the Ness can DJC properly.
Have you ever tried dashdancing to avoid the dair? Gets you out of the way and right back in again to punish the missed attack.
Ness players who spam dair get pwned by this.
If he's DJCing his dairs, watch the timing, and you can actually fit in a grab or fsmash (kirby's fsmash is extremely fast and has great range).
Another thing, if you're playing as Kirby, don't use his grab as a punisher. In fact Kirby rarely needs to grab at all, his throws are terrible and he has better options for most situations.
Also, know this matchup - Kirby gimps Ness very easily, so just get him off the stage and screw over his crappy recovery, and safely return to the stage with your many jumps. This matchup is waaaay in Kirby's favour. Your opponent may just be better at the game than you, or you just don't have enough experience or control of your character to be able to formulate new ways to tackle your opponent's style on the fly. So, keep playing, and use the advice here.
Also post videos if possible! You'll get much more useful critiques that way that will get down to the core of your play issues.