How fair is that some are smarter?

How willing people are to give to others depends on what they like and what they believe. There's (between-individual and between-group) differences in what is considered (believed) to be fair...

...Turning to the effects of the belief variables, our results showed that these too vary significantly across countries. Believing that coming from a wealthy family is important to get ahead in some countries had basically no effect while in others it involved an over 20 percentage point increase in the probability of supporting redistribution. The effect of believing effort to be an important income determinant varied from being statistically non-discernible from zero to decreasing the probability of supporting redistribution with around 15 percentage points. With respect to believing intelligence/skills to be rewarded the results were mixed, with both negative and positive but in most cases statistically insignificant effects.

Isakssona and Lindskog. Preferences for redistribution- A country comparison of fairness judgements. Journal of Economic Behavior & Organization.