For me personally, I have never seen or heard the word "Mikasa" used to refer to oranges. I was born and spent a lot of time in Japan, but I have never heard of the word "Mikasa" except for:
* A company
* Mikasa city in Hokkaido
* The mountain in Nara and the battleship named after it
* The anime character you mentioned
* And lastly, the names of people, and a family of Imperial house of Japan.
I did a bit of googling to make sure my knowledge was correct, but it seems that the word Mikasa is never used for oranges. I think you may have misremembered something.