Is it bad to get your belly button pierced if your super skinny?

Not at all, if you have the right body for it. The only reason a navel piercing would be "bad" is if you have a body that interferes with the healing process. To see if your body is suited for a navel piercing, you should:

• Sit down in a chair or at your bedside, in a relaxed position
• Look to see if your belly button gets tucked under your skin (most girls thing these rolls of skin are "fat," but anyone can have this effect, no matter what they weigh)
• If your belly button tucks under, your body is probably not fit for a navel piercing.

Another way to check is to see if you have a horizontal dent/line that goes across your abdomen (naturally or when sitting). If so, then your body is probably not a good candidate for a navel piercing.