Remember Lieutenant Colonel Oliver North? Testified in dress "greens," as I recall, in a blatant attempt to come across as a super-patriot. Dress greens are a working uniform for an office environment. I don't recall anyone giving him any grief about the uniform.
Vindman, on the other hand, came in in dress BLUES, which are NOT a working uniform. His attire was the equivalent of wearing a prom dress (or a tuxedo) for a job interview.
The essence of his testimony was a rebuke to his Commander in Chief. Not cool.