In large part, yes, but you're also really undervaluing how big Trauma Center was.  Second Opinion was Atlus USA's biggest selling to date when it released.  In fact, since then the only games to outsell it have been Persona 3-4 and Demon's Souls iirc.  DDS was a bomb by comparison, outside Persona none of the SMT games are really all that popular in the US still.

Also, Atlus gained a lot of followers from their championing GBA support, and yes their strong DS support.  They've definitely grown in the west by really supporting everyone and at any possible opportunity.

But gaining a lot of followers on different platforms doesn't really matter if you don't make your games multiplat.  Let's say NIS followed the same route and started making some games for 3DS and Wii, if those weren't just ports why would it be any different from just making those same games on the platforms they already have experience with?  If branching out is the only way a company can gain a great reputation why do Nintendo and Sony's studios have great reputations?  Atlus gained a great reputation and has released on multiple platforms, but I don't think the two are as related as the OP likes to think.

There is a decent jrpg/otaku fanbase on the Xbox 360 in North America (and the Xbox 360 has the hardcore shmup fanbase in Japan and they have a bigger visual novel/dating sim fanbase than the PS3. Though not as big as the PC, where the visual novel/dating sim genres are king). It's just bigger on the PS3 side. When I had bought the Xbox 360, the PS3 was lacking in that department whereas the Xbox 360 had a number of then-exclusive Japanese (especially jrpgs) games. Though now the situation has clearly changed with the PS3 getting a lot of exclusive niche Japanese games.

To be honest, I haven't really tried much of that exclusive niche Japanese library on the PS3. I'm not a Disgaea fan, 3D Dot Game Heroes is a good game but not really something that grabs my interest (I played through a few temples and sent it back. Never was a big Zelda fan but the game got so much hype here that I just had to rent it). And the glut of niche NIS games don't really intrigue me to be honest. Jrpgs are such a huge time sink. I don't feel like playing all of these games. It's much more convenient to play a shorter game (I've started Last Window on the DS recently and it's quite enjoyable) or watch TV/movies to get an entertainment fix (maybe I'll get more into anime as opposed to the jrpgs). Jrpgs sometimes start to feel like work. Though I did rent FF13 recently. I'll see how that goes.

Oh sure, I don't think going multiplatform really would'e made all that much difference for N1.  I'm just saying, a lot of the goodwill towards Atlus comes from their support of systems other than just PS2.

Those two words ARE NOT interchangable!