It would be easy to mow that many yards but finding that many yards is the hard part. Lots of competition, at least in my area, for landscaping services.
Working for $20 per lawn ($80 per month) will usually beat out the competition. If you are going to go that low, though, you need to find multiple clients on the same block. That way you can knock out 2 yards at once in like 30-35 min. Keep commute times down and work out a schedule where you do a different neighborhood every day, and you can just make a ton of money in a summer.