This is a giant limitation for those who work with Sketchup. I don't understand what's changed behind the scenes, but previous converters in previous Max releases have worked fine.
Since updating to Corona 6 hotfix 1 and Max 2021, no imported sketchup models have been able to convert physical to corona mtl. The workaround mentioned is as good as applying a blank material to your entire model, which obviously ruins the possible hundreds of materials and maps previously set up in Sketchup.
My "workaround" to this huge problem now is to use Max2021 scene converter and set the rule to convert Physical mats to Vray MTL, then run the Corona Converter to convert to corona mtl.
Hope this gets fixed ASAP.