Does the sentence "Aren't they pretty!" mean "They are pretty."?
With the exclamation mark, it does mean that the speaker thinks "they are pretty" and expresses it out.
If it were written with a question mark ("Aren't they pretty?") it would be a question, still suggesting the same thought, but perhaps looking for confirmation from the listener.
Note, however, that in some cases the negative question is a request for confirmation of a negative belief - which has the opposite meaning. For example, "aren't you coming?" may suggest that I suspect you are not coming.
You can find a longer discussion here.