I have my smart phone without any protector and the condition of it's screen is very bad. It is full of scratches. Is there any hack so that I made clear my mobile screen?
It is fairly easy to polish plastic, but polishing glass takes many hours using progressively finer abrasives.
That said, you might try covering the scratches with a plastic that approximately matches the index of refraction of the glass. Kits to do so are sold to repair chipped car windshields. Note that the repair is far from perfect: it may still leave visible scratches or lumps on the screes and will be more easily scratched, in the future, than glass.
If the screen is other than ordinary glass (e.g. sapphire or Gorilla© Glass), then neither polishing nor scratch-filler may be feasible, because of the hardness, surface finish or index of refraction of the screen.
Obviously, a clear screen protector can prevent this issue.
No. Your best option is to replace the screen.