On Sketchfab model pages, deleting comments is just as easy as making them.
There are two options to delete comments:
Delete your own comments on others' models
Delete comments made by others on your models
Regardless of the option, to delete a comment on a Sketchfab model is simple.
All you need to do is find the comment on the model page, hover over the gear icon to the top-right corner of the comment you want to remove, and click the Delete link.