Whether she is still technically a princess or not is up for debate but I'd argue that it doesn't really matter if she is or not, there are other reasons to call her a princess.
Out of respect
Lor San Tekka explains this point quite clearly in The Force Awakens, it is still respectful to call her a princess even if the place she was a princess is now gone.
Poe: Well, because of you now we have a chance. The General's been after this for a long time.
Lor San Tekka: "The General." To me, she's royalty.
Star Wars: The Force Awakens
That's what they know her by
It would be odd to instantly call someone else and against your nature to instantly call someone else by another name. Heck I still (it's only been a month and a half!) refer to my wife as girlfriend in my head and sometimes use her maiden name. Things take time to get used to and adjust to.
In addition to this point it is easier to refer to someone as their common and most well known name than the new version of it. Tarkin does this in ANH when alerting people about her because it is what they know her by and princess is quicker than Leia Organa. It just makes communication easier initially.
Tarkin: The princess! Put all sections on alert!
Star Wars: A New Hope
In Han's (and sometime Vader's) case to mock
One of the best cases I think is the quote you use in the question, Han uses princess to mock her, note how he often emphasises it when talking. As he grows fonder of her this changes to be more playful but still used in a jokingly kind of way.
Han: Well, Your Highness, I guess this is it.
Leia: That's right.
Han: Don't get all mushy on me. So long, Princess.
Star Wars: The Empire Strikes Back