Eh. You make fine points.
They leave a bit of a bitter taste in my mouth (I am sometimes a bit swayed by the Just World Fallacy), but you make fine points. I can't argue with that.
Two things are worthy of note in-story, however - and they are quite independent even from my char playung Judge and - I think - way more realistic.
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The mermaid that got (painfully) pranked has a sister who may well not like knowing that her sister got mistreated by their hosts.
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Ricky's parents, should they find out what is being done to Ricky, will likely be downright furious.
These two facts aren't necessarily "karma" so much as "Jeff doesn't always clean up after himself when being a jerk, so he may well be found out by people who may well react in their own ways". In the mermaid case, the mermaid outright KNOWS Jeff did it and Jeff did everyone the favor of leaving evidence - fish food in the pool.