From what I understand, if it’s on Pinterest and involves pokémon, there is a very good chance that it’s actually not a good idea. (The same can be said for Instagram, particularly those involving pokémon diets. I’m just saying, readers.)
On a serious note, while it’s okay to decorate your dwebble or crustle with air plants (so long as you remember not to mist them directly on your pokémon), it’s not okay to place any plant that actually requires planting. Strangely, it’s not because you would need to water them. (Actually, dwebble and crustle can tolerate mild amounts of water themselves, and this is often recommended to help keep their shells clean.) It’s actually because in order to convert them into adequate planters, you’ll need to dig holes into their shells, which is something you often don’t see on Pinterest. Not only is this extremely difficult, but it’s also often distressing for the dwebble or crustle.
This is especially true for dwebble, which are often very particular about the shells they choose and carve, and they’re often too docile to voice their distress to their trainers. Unfortunately, because they often don’t vocalize that this is a problem, dwebble basically lead a constantly stressed existence, which greatly reduces their lifespans. Dwebble that have had this treatment done to them don’t often live past a few months if they don’t have access to more rocks, and if they do, because their trainers don’t understand that this is a problem, they’ll often live for only a year or two after that (as changing rocks may relieve stress temporarily but will only slightly delay the inevitable if they have to do so constantly).
Crustle are a bit more tolerant, however, but this still doesn’t mean you should drill into their shells to make a planter, as the added pressure of you doing so may inadvertently crush your pokémon. It’s possible to lay a thin layer of dirt on top, however, but keep in mind that this may only hold a few small plants—and that this layer should be thin. Additional weight may also be a constant source of stress for a crustle.
In short, it’s not really a good idea, I’m afraid. There are ways to do it with a crustle if you’re very careful (and plant something lightweight, that doesn’t require water), but even then, the risks involved with setting such a thing up are honestly not that worth it. Also, never do this with a dwebble.
On the positive side, air plants are very easy to care for, and many dwebble and crustle adore them, not only because they make very smart decorations but also because they’re a convenient snack. (You may wish to have a handy supply of them or to train your dwebble or crustle to not eat them whenever your back is turned, on that note.)