Ah, I don't think this png will work as a C3 SpriteFont source as is. Each character needs to be in it's own cell with a fixed cell size (e.g. 16x16), it should be left aligned (e.g. character should be set to start on the left side.) In your example, each character has a variable cell size (width varies.)
If the characters have a fixed cell size and are left aligned, the spacing data can be used to place the characters with the appropriate 'width' between each other, so it won't be monospaced.
Here's Good Neighbors:
Here's the correct style of formatting (notice the 'I'):
You can fix it though, edit the image so each character is in a fixed sized cell, left align the character and then use spacing info to make the spacing correct.