Generally speaking, yes. Most modern camping coolers come with lids that feature the same multi-layer insulation as the main body, giving it a good level of solidity. There are of course exceptions to the rule, such as multi-hinged lids that tend to be somewhat thinner. When using your cooler as a seat, pay attention to how much the top panel is flexing against the weight. Constant pressure in the center of the cooler lid can over time lead to warping, which may affect the overall seas and thus lead to loss of overall insulation.