https://fcache1.pakwheels.com/original/4X/9/b/3/9b340712ea59fcca0e663af930dda1d059d38974.jpg
That makes everything crystal clear.
Your bike doesn't seem to be tuned properly. The major cause of carbon build-up on a plug's electrode is incomplete combustion. There are various factors which may cause an incomplete combustion but in your case; the engine is running WAY too rich hence the dramatically enhanced fuel consumption and the carbon build-up on the electrode.
The way I see it; your mechanic has messed up your carburetor setting so if I was you; I'd start by taking the bike to a reputable, competent mechanic and get the carb sorted out. And don't forget to discard this plug, it's worthless now.
I'm 99% sure that it'll solve your problem.
Best of luck!