Although it’s impossible to diagnose a plant problem from a photo alone, especially of one leaf, this does not look like a pest or nutritional problem. Pests (bugs and larvae) do three types of damage: some chomp on leaves making holes, some scrape leaf tissues from underneath leaves creating a paper-thin or “skeletonized” look, and others pierce the leaf with tiny “soda straw” mouth parts making stippled leaves that might be contorted. Your leaf doesn’t look like any of these.
The edge that’s tan and brown looks like fungal damage – most fungus grow when foliage is frequently splashed with water. Water mint that’s in the ground once a week and water a pot of mint every two to four days depending on the weather. Try to water in the morning when the plant has all day to dry, rather than in the evening when it will stay moist all night so fungi can grow.
It is possible that these spots were caused by something hitting the leaves – from one leaf it’s impossible to tell. It might have been hot water from a sun-heated hose, a cleaning product used in the area or a garden product such as fertilizer in the water. Clip off the worst of the leaves and alter the watering as recommended and see if that solves the problem.
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