Smith is a *great* actor when he wants to be, he has proven this many times. I have no problems with his casting. Tarantino shouldn't have much trouble in getting a great performance out of him.
If you can't see Smith's acting talent, then it simply means you're not capable of judging acting skills objectively, and your judgement is clouded by other issues ("His movies suck! His music sucks! All the stupid kids in my school like him, which means he sucks!)".