You are naturally disorganized and messy with things.

It means that you are an untidy person who doesn't clean up after yourself (for example: in the kitchen)

The term "at heart" means "in essence" or "to the core of being". So, to describe yourself as anything 'at heart' means that it describes the very core of who you are as a person. "I am a lover at heart" "I am a nerd at heart" or, in this case "I am a very messy person at heart"

If someone you know sent this to you, it's meant to be humorous. I laughed.