No. Absolutely not. In fact, a vaginal delivery is going to be safer for them. Comparing a woman who is obese to a woman that is not obese, a woman that is obese has a higher risk of very serious complications from a C-section, so it would be better to have a vaginal birth. Unfortunately, the rate of cesarean is higher in women who are obese.
We don't fully understand all the reasons why. Women who are obese are more likely to have larger babies and that could increase the risk for cesarean delivery. They're also more likely to undergo an induction of labor. In absence of those two indications, the risk for C-section is higher in women that are obese.