Is there a version of MultiMarkdown Composer for iOS?
Currently, there is not a public version of Composer for iOS. There was previously a limited beta, but it was shelved because further work needed to be done for the Mac first (which became Composer v2, v3 (beta only), and now v4).
As of September, 2017 Composer v4 for the Mac has been released. I anticipate additional work on it fixing any issues and adding a couple of features. Once that is done, I plan on turning attention to iOS.
That said, the code used in v4 for the Mac has been dually tested and developed to work on iOS, so the core is already there (e.g. the editor and preview). It's the other stuff that needs to be done (file management, sync support, etc.).
I can't promise a timeline, however.