Michelle Duggar is indeed an excellent example of reproductive fitness. However, the idea of Darwinian Fitness does not place restrictions on outcomes of the reproduction. The definition of Darwinian Fitness is the relative likelihood of an organism to contribute to the gene pool of a population, which both Octomom and Michelle Dugar have done to a higher degree than the average human organism. The methods used to calculate Darwinian Fitness do not take other variables into account, such as fertility drugs or developmental disabilities. We must be careful to not mix up the idea of Darwinian Fitness (or reproductive fitness), as described above, with the idea of a more generalized general fitness, or an organism's suitability to survive in its environment and produce viable offspring. In the sense of generalized fitness, I would agree with you that Michelle Duggar would be a much better example, representing an organism that is able to reproduce more efficiently than other members of the population. In the context of the ideas surrounding Darwinian Fitness, however, I would tend to disagree and posit that they are both good examples of reproductive fitness, only because of their contribution to the gene pool, which is the only consideration the Darwinian Fitness calculation takes into account.