Based on Google's tpl-mode for ctemplates, this major mode adds support for Mustache's more lenient tag values and includes a few commands for your editing pleasure.
In your shell:
cd ~/.emacs.d/vendor
curl --location -O https://raw.github.com/mustache/emacs/master/mustache-mode.el
In your Emacs config:
(add-to-list 'load-path "~/.emacs.d/vendor")
(require 'mustache-mode)