Twinkle Twinkle Little Star
Come up with a situation where n.air is the BEST option, and ill be sure to start using it. >.>
See the above post. N-air works suprisingly well against Phantasm/Illusion, but not in every instance. I use it right before Fox or Falco might use Smash-B - and if they don't, fast-falling into an L-cancel is quick enough to allow you to then move onto another edge guard option, such as d-smash.
N-air is also good as sort of a quick, light hit which can either lead to another attack (such as U-tilt mentioned by Copy Cat), but it can also work as "filler" in a combo. Using a N-air after knocking an opponent into the air can distrupt your opponent if they're DIing either for a juggle or for a stronger hit. When they drop back down in front of you, the N-air has little lag and you can go for jabs, tilts, and grabs.
I also like to use N-air as "area control". Although it has low priority, it's almost as big as Kirby's entire body and it can have a hit box for up to 24 frames. This lets me use it above and/or in front of someone to "guard" that area, especially when I predict ahead of time that they want to move into that area. Kirby's floatiness combined with the ability to fast fall it quickly with little lag can let me "drop" it onto people's heads because it doesn't look threatning. Full Jumping for this works best.
What n-air
doesn't have is range and power, which are the two qualities of an attack that people deem most important, and thus overlook any other use for "bad" attacks. (The exception to this is that it seems to have some sort of odd priority over Falcon. Try it out and see, it can often get through The Knee - but don't over use it).
In the end, Twinkle Star is best reserved for players who know Kirby's attacks very well and know when they can get away with it and when it's more prudent to go with a different attack. What would make this attack more useful is if it had multile hitboxes, or at least a "landing strike" you see with the D-air and F-air.