A TikTok prankster's getting rightfully condemned for dumping a tub of cereal on the floor of a NYC subway train, including by the MTA ... which called the stunt "despicable."

The stupidy played out like this -- TikTok star @fckjoshy got on a train heading to the Bronx Tuesday with a box of Fruity Pebbles and a gallon of milk, and proceeded to mix them in a big plastic bin ... like it's his normal routine.

Amusing enough, but then he stood up and "accidentally" dropped it on the subway floor so it spilled everywhere ... causing passengers nearby to scurry away.

The TikTok moron narrated his video of the giant mess he created, saying ... "Someone was like, 'I cannot believe this is happening.' I was like, 'I can't believe this is happening again.'"

Of course, the fact it happened at all has sparked justifiable outrage not just from random folks on Twitter, but the MTA ... who called fckjoshy's prank "A new low."

It added ... "Pulling a prank on essential workers in the middle of a global pandemic. And making essential workers clean up your mess. Despicable." Not to mention the ridiculous waste ... as millions of Americans have to wait in hours-long lines at food banks