I don't think it ever aired in the US, because I have never heard of it before
Same. I’ve never heard of Mix Master until now, but it seems like an interesting show. I might check it out. It might not have been popular in the USA but it certainly popular somewhere considering that the MMORPG website is still around.
Surprisingly this anime’s online game; Mix Master: Master of Cards is still up and apparently running! They also have a discord server. The graphics look very, very old-school 2.5D, but might be cool to check out sometime.: www.playmixmaster.com/
I am currently scouring Anime News Network but I am wondering if it was one of those Western Semi-Animes similar to Voltron and Avatar and Totally Spies.
FYI, Voltron is not a western semi-anime. It is 100% Japanese and originally known as GoLion, albeit it was edited and episodes ran in a different order for the westernized Voltron version.