With what sequence of increments?
If the increments are chosen to be: the set of all numbers of the form (2^p * 3^q), that are less than N, then, yes, the running time is better than quadratic (it's proportional to N times the square of the logarithm of N). With that set of increments, Combsort performs exactly the same exchanges as a Shellsort using the same increments (the "Pratt sequence"). But that's not what people usually have in mind when they're talking about Combsort.
In theory...
With increments that are decreasing geometrically (e.g. on each pass over the input the increment is, say, about 80% of the previous increment), which is what people usually mean when they talk about Combsort... yes, asymptotically, it is quadratic in both the worst-case and the average case. But...
In practice...
So long as the increments are relatively prime and the ratio between one increment and the next is sensible (80% is fine), n has to astronomically large before the average running time will be much more than n.log(n). I've sorted hundreds of millions of records at a time with Combsort, and I've only ever seen quadratic running times when I've deliberately engineered them by constructing "killer inputs". In practice, with relatively prime increments (and a ratio between adjacent increments of 1.25:1), even for millions of records, Combsort requires on average, about 3 times as many comparisons as a mergesort and typically takes between 2 and 3 times as long to run.