Not necessarily. Mosaic is anytime two base colors or patterns are together on the same bird. This is normally impossible. A bird is either ash-red, blue, or brown. And either t-check, check, bar, or barless. They can carry recessive genes but only one base color and one pattern is supposed to show.
But with mosaics, something happened to where that rule was ignored. The two colors/patterns can be shown anywhere on the body. I have a mosaic who is an ash-red bird with a large blue check patch on his(her?) lower neck and a bit into his back. Some birds will be split down the middle, and some will just have more random spots.
Almond can pretend to be mosaic because they have a base color, and then have flecks around the body. Sometimes these flecks come out quite large, and make us think it is a mosaic, when really, there was no genetic 'mistake'. It's just the way almond makes a bird look. Just like spread makes a blue bird look black, and how indigo makes a bird's bars look like rust.
We're still not completely sure how mosaics work and if there's more to it than just embryo fusing or two sperm+one egg. Never know, may not be a mixup in the genes at all, but actually a complex combo of genes that we haven't figured out yet.