Textures as in 3d textures:
No simple tool exists for several reasons including the "common" files need not be so common (NBSMD is the nitroSDK format and I will probably go on about it, many other "major" games use their own tweaks and entirely new formats), there are tools to view: (search for NSBMD tool) guides to the format: http://kiwi.ds.googlepages.com/nsbmd.html
and perhaps http://www.romhacking.net/docs/%5B217%5Dgbatek.htm#ds3dvideo
and nitro SDK sections (no link for obvious reasons)).
Most DS 3d graphics are compressed and/or buried in subfiles (NARC and the like, indeed NSBMD tool will require it to be in base format). A few things like Animal crossing have had importers built for the save editors http://www.aibohack.com/nds/ac_mapedit.htm
and the pokemon crowd also have a handful of game/series specific tools and in something that is entirely unrelated there was a viewer for metroid way way back.
Editors are hex editors or nothing really (I do not even think there is a simple "parts lister" as it were), you have the format and so it is not that hard to do considering most objects are not that complex.
Most hacks I see are simple file system hacks (replacing one with another) or size (scale) hacks (we had a big wheels hack for Mario Kart somewhere around here the other month). Before you go looking for the nitroSDK know that it uses "standard" PC formats and only converts them in the endgame.
Many many 3d objects use material colours instead of textures (the DS 3d system is quite good but compared to what we play with elsewhere incredibly primitive) and then have a few shadows or other 3d "features" give the illusion of a texture.
2d textures exist although normally the shift is more towards dynamic items than "true" textures (those rapidly changing colour blocks on Mr Driller are a good example) but not at the same level that 3d play with (is it ever) so most 2d work you will see is a clever combination of sprites, tile arrangements, backgrounds, cutaways, fast changes and other "tricks of the trade" (it has been what 30 years at least now for "true" games and for most of that we have not had the uber powerful machines we have today so coders are really good at this stuff). Take note that more than a few games use 3d but display it in such a way that it appears 2d and vice versa.