Recently spotted mu4e-marker-icons, which adds mu4e icons using all-the-icons.
Although I'm not currently using all-the-icons, it did remind me to take a look at mu4e's built-in variables to spiff up my email. It's pretty simple. Find the icons you like and set them as follows:
(setq mu4e-headers-unread-mark '("u" . "📩 "))
(setq mu4e-headers-draft-mark '("D" . "🚧 "))
(setq mu4e-headers-flagged-mark '("F" . "🚩 "))
(setq mu4e-headers-new-mark '("N" . "✨ "))
(setq mu4e-headers-passed-mark '("P" . "↪ "))
(setq mu4e-headers-replied-mark '("R" . "↩ "))
(setq mu4e-headers-seen-mark '("S" . " "))
(setq mu4e-headers-trashed-mark '("T" . "🗑️"))
(setq mu4e-headers-attach-mark '("a" . "📎 "))
(setq mu4e-headers-encrypted-mark '("x" . "🔑 "))
(setq mu4e-headers-signed-mark '("s" . "🖊 "))