The actual pineapple on the front porch or mailbox is a meme...like the myth of having a large white landscaping rock out front of your house if you're open to swinging.
Rather, the upside down pineapple is a joke that some LS folks use for laughs. You can buy shirts, hats, beach towels, etc., with upside down pineapples and it's meant as a fun icebreaker. The first time we saw them was in Key West, many years ago; we've seen them here and there ever since. We've actually ordered a few tank tops in advance of our fall trip to Desire Pearl.
If you are so inclined to participate:
https://www.amazon.com/s?k=upside+do...s_ts-doa-p_1_6