Is the complement of the language generated by $S \to aSbS|\epsilon$ context-free?
The first step is finding a simpler description of the language generated by the grammar $S \to aSbS \mid \epsilon$.
The second step is deducing a description of the complement, which is the language you're actually interested in.
Finally, in order to show that the language is context-free, you can consult our reference question on this topic.