Zelensky was asked BEFORE the meeting to please wear a suit to the Oval Office.
So right off the bat that disrespect set the tone.
Your assuming he brought one with him, OR, that he even owns one, right?

IMO, culture clash played a part in all of this. He has always dressed pretty much the same. As "one of the people." When Obama was President, numerous Foreign Dignitaries visited without wearing a suit. Never heard anyone criticizing them. Republicans or Democrats.
I always considered the White House to be the "House of the PEOPLE," NOT the House of proper dress code. Shoot, one of our Presidents even placed a huge wheel of cheese in the foyer of the White House and invited the PEOPLE in to have a snack. Dress Code not imposed.
Pomp and Circumstance has a time and a place for sure, but I'm not sure that kind of meeting requires it for all Parties to be "civil" and act like adults. Given Trumps actions throughout his public life, not wearing a suit seems hardly worthy of the wrath of Trump and the Junior High Student sitting next to him.