Cora calls Mr. Brown "Mr. Brown," first because she only recently found out that he is her father. She and Mr. Brown have known each other for all of her life, he lived next door to her and her mother, Mable "Madea" Simmons, but no one ever clued her in to the fact that he was her father.
The other reason she calls him "Mr. Brown" is the fact that in the South, we call all of our elders by Mr. or Mrs. Therefore, Cora was used to calling him "Mr. Brown," so she just took that trend along with her when she found out that he was her father.
In an interesting and *shocking* turn of events, audiences found out in Tyler Perry's last move, "Madea's Big Happy Family (2011)," we learn that "Mr. Brown" may or may not be Cora's father after all (I won't spoil the surprise). Madea, Cora, and Mr. Brown all take a trip to the Maury Povich Show to find out the truth....
It is pure Tyler Perry silliness, all in fun...:)