If you are, then your chances of having bad breath are greater. This is because mouth breathing dries out your mouth, letting the bacteria grow and the odors escape.
Some people are mouth breathers because their nasal air flow is somehow obstructed.
This can also cause odor from you nasal passages. If you are a frequent mouth breather because of allergies, then some of the nasal discharge may find its way to your tongue, where it can be a major cause of bad breath.