Is it legal to download mikumikudance?

Yes, it is legal to download MikuMikuDance.

You see, MikuMikuDance is a 3D software that had leaked into the US, Japan released it free in Japan, so when it got to the US, they thought it would be free for us, or they just didn't know at the time, and millions of people already got it for free.

So Yes, it is legal to download MikuMikuDance