You can use Mer with mainline kernel if you want, but really then the question becomes "Why use Mer?".

Mer for the most part does not have a huge amount of packages available, and most of them are old. If you want something that's reasonably bleeding edge and you don't want to maintain a lot of stuff yourself then you'd pick something else. In this case that something else is Alpine Linux, but there are other options.

If however you want to use Android kernels and drivers, then those kernel versions are old and you usually need old packages to go along with it, which is where Mer becomes useful.