"Your mouth smells."
The word "bau" by definition is neutral and can function as a noun (the quality that you sense through your nose) and as a verb (to exude a certain smell). You have to add a qualifier to say if the smell is good or bad.
However, in everyday use, just like the sentence above, this word used by itself it may imply that the mouth smells bad. If you need to avoid this negative connotation, you can use the noun "aroma" (which has a positive connotation).