How does carbonic acid cause acid rain when Kb of bicarbonate is greater than Ka?
What is the ${K_a}$  of carbonic acid? Note that sources differ in their ${K_a}$ values, and especially for carbonic acid, since there are two kinds - a pseudo-carbonic acid/hydrated carbon dioxide and the real thing (which exists in equilibrium with hydrated carbon dioxide but in a small concentration - about 4% of what what appears to be carbonic acid is true carbonic acid, with the rest simply being $\ce{H2O*CO_2}$. 

Either way, I find that the ${K_a}$ of the mixed carbonic acid is about $4.2 \times 10^{-7}$, which is greater than $1.0  \times 10^{-7}$, and this implies that a solution of carbonic acid alone should be acidic no matter what. This suggests to me that your numbers are wrong; would you mind sharing your numbers and their source if possible? 

EDIT: I see that you have updated your numbers. Nonetheless, I believe that your ${K_a}$ for carbonic acid is wrong; that number looks suspiciously like the ${K_a}$ instead for hydrogen carbonate ion (or the bicarbonate ion). According to Wikipedia, the ${pKa}$ of carbonic acid, is 6.3 (and this is taking into account any aqueous carbon dioxide). Convert this to a ${K_a}$ value and we get about $5.0 \times 10^{-7}$. This is in-line with the value I obtained from a copy of Daniel C. Harris' Qualitative Chemical Analysis.

EDIT 2: I think you've realized your mistake; as you say, the values are for $\ce{HCO_3^-}$, which is the hydrogen carbonate ion.