Heraldry was of high importance among noble knights. It's not surprising that Oda made them look like parade floats. In fact, If Oda really wanted to be historically accurate, they'd look even more elaborate. The only thing he should have added to their attire are family symbols. Noble knights always represented their lineage by wearing house symbols and colors. They even had something called the rule of tincture which was a widely respected practice of avoiding similar colors and using high contrast to remain distinct. So having the Holy Knights sport some all black and red regalia wouldn't fit their character. They want to stand out, like peacocks; they have no interest in being conventional, even if they look ridiculous in the process, as long as they catch people's eyes then their ego is satisfied.