Lucas/Ness is a hard matchup because you have to be super careful about using Din's, something that you'll basically abuse against everyone else.
One note on using Din's though, you can still use tactics where you explode Din's relatively close to you (so it'll stop someone as they make their approach without getting them to close to actually hit you if they shield). Most Ness/Lucas players aren't going to pull their absorb move out if they run the risk of taking more damage from another move. Just keep in mind when its safe to do this.
If you're feelnig too predictable, then you need to vary Zelda's defense. Her D-smash, F-smash, Neutral A, F-tilt, and D-tilt are all very good for protecting in front and she's basically covered from air attacks with the up-smash/up-tilt.
The speed, startup/end lag differences, and hitboxes on a lot of Zelda's moves are relatively similar but slightly different. What that means is you need to use a variety of her attacks. If you're just using the smash attacks and maybe 1 other move, then all your opponent has to do is wait and counter because he knows the timing of when to hit you. I say that because nearly every Zelda player starts off as a smash @#$RE and over uses them without touching the tilts.
That being said, dont' just throw moves out there. Watch the spacing and use each tilt/smash in the correct situation. Just spamming attacks isn't going to help you.
One last thing, against Ness/Lucas don't make stupid aggressive moves just because you can't bait him in closer (with Din's). There are moments when you need to play Zelda aggressive, but don't do it just because you can't force him to come to you right away.
