Alternatives for "overnight sensation"

a fad or a craze refer to fashion, games, dances, toys, etc. that generate short-lived popular enthusiasm, but they don't usually refer to people.

I therefore suggest "an ephemeral bang" or " an ephemeral hit".

• ephemeral - "Lasting for a markedly brief time"

• hit - "A successful or popular venture"

• bang - "a sudden striking effect"

I'd suggest that the person is having his or her fifteen minutes of fame.

fifteen minutes of fame PHRASE: A brief period of fame that a person enjoys before fading back into obscurity. -ODO

It's credited to Andy Warhol, who included the words "In the future, everyone will be world-famous for 15 minutes" in the program for a 1968 exhibition of his work at the Moderna Museet in Stockholm, Sweden.

Nine Days' Wonder or, in the case of a pop or rock act, One Hit Wonder certainly convey the idea of sudden, short-lived fame.

The only weakness is that neither can be applied to someone who acquires celebrity instantly but goes on to more permanent fame.

They are a here today, gone tomorrow star, whose fame lasts whilst are flavour of the moment.

here today, gone tomorrow saying said about something that lasts only a short time:
A lot of new internet companies are here today, gone tomorrow.
— cambridge.org

And

flavour of the moment Something that is prominent in the public eye for a short time then fades out of interest. Originally a term of approval for something that was up to the minute and desirable. It has been used ironically from the late 20th century to pass disdainful comment on things which pass out of fashion quickly; for example, the "one hit wonders" of the music business.
[...] The phrase dates back to 1930s USA and originated in the advertising slogans of ice-cream companies there.
— phrases.org.uk