From what I understand, in Dark Sun, Defilers can destroy the plants around them to make their magic more powerful. If a Defiler from Athas somehow ended up in another material plane (don't care which, don't care how) could they use the abundant life energy to power their spells? Or, does Defiling only work on Athas due to the Weave or something else?
They can only use Defiler Magic on Athas, however...
They could still possibly use magic, but it will be from an alternate source, as they will find themselves unable to harness the same energy from the environment as they did on their homeworld. It's possible they might need to 'attune' themselves to the way magic functions in their host realm, and relearn spells. Because a Defiler is defined by their mentality, (having no regard for how their magic effects the environment,) they would still be considered a Defiler, and might attract the patronage of evil powers for spellcasting.
According to the official Dark Sun 3rd edition pdf
On Athas, the energy for wizardly magic doesn’t come from some extradimensional source as it does on other worlds, but from the living environment itself. It provides great power to those who can gather and shape it, though the cost to Athas can be beyond measure. In recent times wizards have emerged who have learned to draw energy from alternate sources that have no impact on the environment,
The fandom wiki entry on Defilers, from 2nd edition states that
On Athas, magic and the ecosystem are irrevocably bound —no one, not even a wizard, can affect one without affecting the other.
All wizards must decide at the beginning of their careers whether they are trying to work with nature or without regard for it. In Dark Sun, if they decide to work without regard for it, it means the wizard must be a Defiler.
So, magic on Athas is unique. The natural world has become so infused, that it's a viable source of power to be cultivated, harnessed, and exploited.
It is likely that they can use it elsewhere, subject to DM's approval!
The 2e version of defiling and preserving is mechanically complex and hence during the 3e period, WotC's official Dragon magazine provided updated defiling rules in the issue 315, published in January 2004. On page 35, we read:
You can easily use the defiler magic system in a game that doesn't take place on Dark Sun's world of Athas ... If you use these rules in a campaign world you've created yourself; you should make some adjustments ... As presented in this article, the defiler magic system assumes that vegetation is hard to come by and that defilers live hunted or secret lives. If you eliminate either of those factors, you might make defiling magic more powerful, so you might want to add some more costs to the system.
So it feels like the authors were not necessarily forbidding the use of defiling magic outside of Athas. This general "mood" is also supported in some other publications, as you can read on the following related (reverse) question: Does non-Athasian magic work on Athas?