I wouldn't recommend the "terraria" approach either: although the overall shape of the island is interesting, the non-organic squariness of the tileset is too obvious. I'd just say "if your tileset can be used to tile that map, it's not missing any corner tiles".
You should really increase the contrast though.
Should he ?
I mean, dirt is meant to be a sort of placeholder, with no gameplay potential, right ? It should not distract the player from important things (platforms, ennemies, bonus ...) by being overly contrasted, despite checking a more contrasted version would help us evaluating the quality of the pixel clusters...
Or did you mean "contrast on the green 'grass'" only ?