Specifically when you land on the ground after using cyclone slash in the air, and during the animation of cyclone slash, the boss screams. Is this a major glitch? I can't find anything else that you can do, other than sillily slide across the screen.

Which boss are you referring to?

You can do the same thing with cyclone slash during the opening animation of the Black Egg Temple door, and it's not considered a major glitch there. You also don't have control of your character while doing this, and there are afaik no known uses other than looking funny.

During the start of the winged hornet fight, it is possible to perform this glitch and slide backwards towards the spikes, allowing early control.