Mint green is a pretty assertive color, isn't it :)
A couple of things you can try.
One would be using a darker or more muted shade of green, to connect with the mint but tone it down a bit.
Another way would be to use some large artwork that won't be harmed by water (or is cheap enough for it not to matter) to cover a good chunk or chunks of the wall area.
A darker or more muted shower curtain which stays fully or partly closed will give you a large area of darker color to tone down the walls.
If all else fails, maybe you could repaint - it's a bathroom, so probably not very big. Even repaint just some walls and leave the mint green on one wall as a feature?