Mir is neither of these, it was created by one group, is intended to primarily serve the needs of that group, and is not accepted by any recognized standards-setting organization. Wayland, on the other hand, is a standard, specifically a standard accepted by freedesktop.org (the recognized standards-setting body for FLOSS software), and was created with the goal of being cross-distribution.
What you are thinking of is "de facto standard", which is not the same thing.
In this regard it is more like the W3C rather than the ISO, in that it establishes standards that apply to a certain class of software, rather than to anything and everything.
As usually nobody (me included) really knows what that really means, unless we get a Wayland dev to talk about thread safety at large, about the pros and cons of having different parts (or all) of the wayland stack thread safe.
Notice, back in Oct. 2012 there were thread-safety patches merged into Wayland before 1.0.
Then there's recent thread related additions like this one from March 2013.
In short, nobody (not the Wayland nor the Mir devs) knows for sure all the pros and cons of the whole thread-safety shebang: what parts must be thread safe, which should not, and which ones should provide thread-safety optionally - until we have a fully working desktop on either Wayland or Mir with latest technologies (KDE/QML, Gnome/clutter etc) fully ported to see how they work and which approach is the best. Note to non-programmers: being thread safe isn't always necessary or a good idea, though often it is.
Regardless, both of them will be a lot better and simpler than X11. Also, Mir contributed to Wayland indirectly by becoming a competitor and hence accelerating the development of Wayland, but Canonical could have accelerated it even faster by assigning those 8 devs to work on Wayland (from 2010 as promised!) rather than diluting the effort on a (very) similar solution.
"De facto standard" IS the standard. It is one only when everyone agrees to follow it; who cares what is written on paper if it does not coincide with reality? I can't remember Debian abandoning dpkg because of LSB-compliance issues.