Lady GaGa -- C-Listed by Southwest

Ding! You're now free to fly amongst the commoners.

She had just opened for the New Kids On the Block, but Sunday night, dance diva Lady GaGa got stuck dealing with all the crap that every other miserable airline traveler has to deal with -- carrying her own suitcase and fighting for a seat on a crowded Southwest flight.

TMZ spies spotted a sunglass-clad GaGa -- aka Stefania Gabriella Germanotta -- waiting to board a night flight from Vegas to Burbank on Sunday -- in the dreaded Group C!!!!!!

By the time GaGa and her motley crew of dancers and hangers-on boarded, hardly any seats or overhead space was left.

Just tragic.