I just read a bit about Yocto and it doesn't seem to be in contradiction with Mer. As far as I understood, Yocto is project to provide an ecosystem which can help to create Linux distributions for the embedded world, not a distribution in itself. And Yocto targets embedded Linux, but not only mobile phones.

Mer is a distribution which targets mobile phones.

I would say with Yocto you can create Mer...which already exists. Didn't really see the added value in this particular case, but I didn't too long to read about Yocto.