While mileage is usually a good indicator of cosmetic wear like leather/paint condition. Higher mileage cars that are well maintained are usually better in terms of mechanical condition.
Cars are made to be driven. The longer a car sits around, the worse condition it is in. Tires go bad when they aren't driven on, the engine and transmission need oil to lubricate them and keep them in good condition - oil doesn't pump when the car is not on, batteries die when a car is not driven regularly etc.
So I'd say lower mileage is better but too low mileage is bad. I'd like to see a car driven at least 5000 miles a year but no more than 10,000 miles a year. If it is a regular car like a Camry or Civic, I'd say up to 15k miles a year is acceptable.
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