Привет, Michael.
By the looks of it, that is mainly due to the lighting in Sketchfab. You can try a different HDRI. Maybe you also need to tweak the shaders in Sketchfab a bit; as far as I remember they use a ‘translation’ of Blender materials and that can be a bit off. (Or a lot!)
I tried uploading to Sketchfab and couldn’t get the materials to look even halfway decent, but I must say I gave up quite soon and avoid exporting as much as I can; if I even hear the word Export, I put on a blindfold and some loud music and pretend to be an ostrich;)
