Why was "Mr. Raindrop" transliterated into hiragana rather than katakana?
I wanted to confirm that was this done intentionally for a joke (if yes, can you please explain it) or it was a careless mistake.

It is perfectly normal to write furigana (in hiragana) on top of English words for the audience, who are practically all Japanese, to understand and learn how to pronounce.
In addition, furigana is always added on top of English words at any Japanese karaoke in Japan.
So no, it is not a joke nor a careless mistake, just a very common practice.