Lil Tay's startin' a music beef ... blasting comparisons to JoJo Siwa -- and slamming her as a fraud for purchasing her songs.

The controversial rapper took to X after one user jokingly called her Siwa while sharing a clip from her "SUCKER 4 GREEN" music video.

Welp ... Tay didn't take too kindly to the comparison -- 'cause she says she just found out who JS was from the tweet, and she tells fans she doesn't want to ever hear their names in the same sentence again 'cause she doesn't pay other people to write her music.

Another fan tried to placate Tay ... saying they felt the joke actually was more how LT did the rebrand the right way, unlike JoJo.

But, Tay says she gets it, and still isn't happy about their names linking ... adding she takes her music hella seriously -- and thinks JoJo's a fraud who's buying her tracks fully finished.

ICYMI ... JoJo put out her new song "Karma" -- complete with a music video -- last week ... and admitted to TMZ the song got pitched to her -- which she argues is normal operating procedure in H'Wood.

This all came after an unreleased version of the song by Brit Smith from more than a decade ago leaked online ... though JoJo swears someone pitched the song to her, and since it wasn't officially released, she's done nothing wrong.

Seems like Tay disagrees ... 'cause she's using very harsh language to describe anyone buying their music -- and, claiming she's doing her work all on her own.

Worth noting ... "SUCKER 4 GREEN" came out about six months ago and has about 7.8 million views on YouTube -- while Jojo's track's already up to 20 million views in just over a week. So, it seems fans are less concerned about where the song came from.