I agree, with the luxury of hindsight, that it was a rather garish era in UI. But looking back now at his Kai Power Tools (the first I heard of him) it is clear to me he was more responsible than anyone for kicking off that whole era of wacky UI that culminated/ended with brushed metal and Apple's Aqua.
So kudos to Kai for having such an outsized influence on the UI of the time.
Back then we would talk about UI when we meant UX. The KPT plugins had a very odd visual design but the way that you could iterate patterns and processes on an image was unheard of.
People also look at the designs that the filters made and didn't think about how they could be used as selection channels and other ways of manipulating an image. Kai's original work was about using selection channels in Photoshop to create stunning effects and while you could use his tools to create a background or a texture they could also be used to manipulate the colour and tone in images in really incredible ways.
So kudos to Kai for having such an outsized influence on the UI of the time.