In general I'd say it's a bad idea to play jokes with email. Most people use it for important work, and many use it as part of their livelihood (not just for professional correspondence as in finding jobs but as a component of their day to day work). Just... don't fuck with that, it's unprofessional to mess with people. Imagine a car maker doing an over the air firmware update of their cars to disable the signal blinkers because isn't it funny how so many people never signal while driving?
Actually no, the problem is that the joke they came up with is the type that requires UX testing in the first place, i.e. it changes the behavior of a product in a very noticeable and non-reversible (by the user) way. As opposed to something harmless, like a joke announcement or something.
Several years ago Blizzard announced the "Archivist" class for their popular game, Diablo 3. They put a good amount of effort into the joke, with a short gameplay video and everything. But it was a joke, and the class didn't actually make it into the game, and everyone had fun.
However, that completely goes against the point of this joke. It's not a prank if they prompted, "Are you sure you want to send a Minion gif and mute yourself from this conversation?"
Nah, people just click through. GMail has a much better model: mask destructive / un-undoable behaviour behind a short timer, make the action undo-able until the timer is up.
Strictly, the action you start when you click `Send' is reversible for a few seconds, before it actually launches the nukes. But I'd rather call it `delayed' when talking to a human about it.
Delaying is a way to make some non-reversible actions sort-of reversible.
Muting is a much bigger problem.
The new button performed a destructive action (because it blocked the recipient from responding to the email thread) without any kind of confirmation. Additionally, it was placed where it was likely to be hit by accident. (Apparently where the "Send & Archive" button used to be, although I don't have that on my Gmail theme.)
In short, they didn't test the damned thing. Even a half-hour of user-testing would have shown how easily the button was pressed by accident, and how devastating an accidental press was.
And again: it doesn't matter if the change is a joke or not, you don't push up a change unless you've done some basic UX work on it.