Dair is most viable as a punish option when someone fails to cover your landing. That right there will get you the most mileage as it's a great way to get combos started. Having said that, I do have some points to make on the move.
I know most of the people here and on the Discord dislike the move, but I do find myself using it often enough to say that the move is still very important to Kirby's core gameplay. I'm not going to be talking about the combos here. I mostly see it as a means of punishment that helps you tack on extra damage.
Dair as a way to cover standard ledge getup, getup attack, and ledge jump is a good way to get the extra percent that a nair wouldn't give you. The trade off being that a nair would send the opponent offstage over just getting some extra damage with dair -> follow up. Also after tripping with dtilt, depending on your spacing, you can use dair as a tech chase tool. Some characters have short getup rolls and you are able to catch multiple options with your dair even with Kirby's poor air speed. On top of that if you provide enough pressure to the opponent's shield (or your opponent likes rolling into you), you can use dair to cover a roll in while staying at a distance.
The problem here though is that dair relies on you getting a read on your opponent. You can use it to cover multiple options, but only in specific situations and it's not ALL the options. It's just not a move you can use to react to people's options, rather to predict people's options. Dair is a great tool when you're in an advantage state and not so much as when you're in neutral.
Though that doesn't mean it is completely useless in neutral. Depending on what character you are facing, and your opponent's playstyle, it can still be useful. Using dair on top of someone's shield while fading back and landing with the landing hit hitting the opponents shield can (depending on the character) put you outside of their grab range deeming it a safe option. On top of that, crossing up on shield is also a good option (depending on the character) because they can't just shield grab you and from there on need to choose an out of shield option.
Using it in those two ways is a good way to force a reaction from the opponent, but it doesn't come without risks. 18 frames of startup sucks and it can be easily anti air'd if people are ready for it. Also, a player who is highly mobile will spend less time in shield and more time not in your range. While that's mostly the fault of Kirby's movement it attributes to making this move much more difficult to use in neutral.
Anyways that's just my thoughts on it. I know it's just a joke, but I do feel like people are undervaluing the possibilities the move has as a tool. Hup canceling is great too, but I don't feel like it belongs in this post.