The Dallas Mavericks just can't seem to peel themselves off the doormat ... even Southwest Airlines is now taking popshots at them!!

The plane co. dissed the Mavs in an Instagram post on Thursday ... making fun of the NBA team's controversial Luka Doncic trade while defending its new checked bag policy.

In an effort to try to soften the blow of telling their customers they're now charging some of them for checked luggage ... Southwest officials wrote in a carousel, "It's not like we traded Luka..."

The quip had many in the comment section rolling -- even Patrick Mahomes' mother found it hilarious -- but we're sure those at the Mavericks' headquarters weren't cracking many smiles.

After all, they've been taking a proverbial beating since jettisoning their 26-year-old superstar to the Lakers in exchange for 32-year-old Anthony Davis ... with some fans boycotting games and others demanding general manager Nico Harrison be fired.

In fact, things have gotten so contentious in Big D over the swap -- one Mavs backer told TMZ Sports last month he was actually thrown out of the arena for trying to start an anti-Harrison chant.

Things, of course, haven't gotten much better in recent weeks -- Kyrie Irving tore his ACL and Davis is still out nursing his own injury woes -- and we're sure Southwest's latest pile-on has done nothing to alleviate the headaches.