Christopher Jackson, who played George Washington alongside Ariana in the musical back in 2015, gushes over the "world-class artist" ... telling TMZ she represents every facet of tremendous skill and talent to the highest level.

He's imploring folks to find another artist out there who has delivered performances with Ariana's sheer skill -- basically, he's yet to see her do anything but shine.

Christopher's also reminding us the job of a Broadway star ain't easy, so much so they have to always consider the safe route -- but Ariana's got it all locked down cause she's a triple threat in every sense of the word.

He blasts the "lazy hack-ass" writers for the joke, but Victoria Theodore -- musical director for 'The Donna Summer Musical' in which Ariana starred -- is taking a direct shot at Bella Ramsey for delivering the quip.

She says Bella's scripted line was rude, and lumping Ariana in with "actors who think they can sing" was completely uncalled for.

Victoria also noted how challenging it is to sing live on Broadway while dancing and acting, but despite that ... she says it's no sweat for Ariana, because she can perform all aspects of musical theatre brilliantly.

Ariana's immediate facial reaction to the joke has gone viral ... and she later admitted on social media, she genuinely didn't find it funny.

Maximizing the awkwardness was the fact Bella was standing next to Anthony Ramos ... another one of Ariana's "Hamilton" costars.