Not grown this particular mint. But generally, mints prefere cooler weather, some shade, lots of water ... little fertilizer.

They have agressive root system and can become root bound in small pots.
This is the time of year that mints slow down, go down, and even disappear in hot summer but come back later. Spring weather (cool, rain) is the peak for mints.

Agree that it looks like a normal mint plant that hasn't been harvested or trimmed. If you choose, go ahead and clip the sparse stems off at soil line. As Seysonn said--this time of year, your mint should rebound quickly with pretty new thick growth.