Chubby's Surprise Song

Origin

The premise comes from the source material. Gian (Takeshi Goda) in Doraemon is a catastrophically bad singer who nonetheless gathers people in the empty lot to hold recitals — a recurring gag. His singing is drawn as something close to a disaster: in one bit, a tape of it sent to a producer using one of Doraemon's gadgets lands that producer in the hospital. The character setup is documented in the Namu Wiki entry on the character.

The "reversal song" comes from material where that premise flips. Within the source itself there are episodes featuring mirror worlds and planets that are the exact opposite of Earth, and in those worlds Gian invariably sings well. There is one more decisive, live-action piece: a TV commercial for UGA, a Japanese commercial karaoke system, in which Gian appears and sings beautifully. The ad opens the usual way, with classmates dragged to a recital and bracing themselves, but what they hear is a gorgeous voice belonging to someone else entirely, and the room is moved to tears. The song in the ad is titled "Ore no Hana to Shiroi Hana."

Spread

That commercial became the meme's actual raw material. It was made to advertise UGA's audio-processing feature, which makes singing sound better — and because the subject happened to be the definitive tone-deaf character, the ad itself lands as a reversal gag. As the footage circulated repeatedly as fodder for edits on both Japanese and Korean internet, "Gian is actually a great singer" hardened into a standard image.

In other words this meme did not start from fan edits; it is closer to the internet picking up a reversal the advertisement had already completed.

Usage

Applying it when someone written off as incapable turns out to be unexpectedly good.
Referring to a setup that lowers expectations and then overturns them with the result.
Quoting it when discussing a surprising side of someone with a fixed reputation.
Bringing it up as a joke whenever singing ability is the topic.