Is it normal for non-dominant arm to be slightly weaker than dominant?i am right-handed and my left arm/hand is only slightly weaker. i am already very skinny so is this normal?

Sure: A person's dominant arm and hand should be stronger than her other arm and hand, even if the difference is small. The dominant arm and hand does the majority of her activities, so generally ends up bigger than the other arm and hand. That said, if she frequently uses her non-dominant arm and hand for heavy lifting or heavy work, the non-dominant limb could end up bigger.