If you try opening a .md file on Android it says "no viewer available", and offers you to pick from a list of Google Drive, Dropbox, ... It doesn't offer to open it in a text viewer. In fact, I'm not even sure Android has a native .txt viewer.
Android doesn't come with a text editor by default but there are loads of options out there. Same goes for any docx or even PDF if you don't have Google Drive installed.
The fact that a file can't be opened as plaintext in a plaintext editor is not a problem with Markdown, it's an OS problem that effects any file you'd want to open as plaintext.