The DS has no operating system, so there'd be nothing to handle the decompression and manage it successfully. Now granted, on slot-2 Supercards you used to be able to compress GBA games, but decompressing took a good amount of time before the game would launch, and the feature is disabled in newer patcher versions.
Other than that, your main source of reducing filesizes is done through trimming. Look up NDS Tokyo Trim. You won't save huge amounts of space, but it'll help a bit.
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