I know in general plants will have more pungent leaves if slightly stressed. i.e. more sun and less water. The chemicals that make leaves smell are a form of insect deterrent, so if conditions are flush, the plant will just push out lots of weak, watery growth and hope to beat out the pests on quantity. If times are leaner, they will concentrate protective (and smelly) compounds in the leaves they have in the hope insects will go elsewhere. Now, obviously, if things are really rough they are spending all their resources just staying alive and don't have any to spare for insecticides, so it's a balance.
I don't know how much of this applies to mint in specific, it's more general theory, and mint does require more water as a baseline than standard herbs like rosemary or oregano, so there may be quite a bit more nuance to it.
