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Kiara Mia isn't bailing on 49ers QB Jimmy Garoppolo after one bad game, because she says he "radiates greatness" ... and you can't hold him back for long.

We got the popular porn star at the Century City Mall in L.A. Friday night and asked about Jimmy G's poor week 1 performance -- in which he threw 3 interceptions against the Vikings and lost for the first time as a starter.

She tells us it's no biggie, and we can bank on Garoppolo bouncing back Sunday against the Lions because he's a star ... and that's what stars do. Kiara better be right ... otherwise, her Super Bowl prediction will be looking a bit dicey.

Play video content 7/19/18 TMZSports.com

We broke the story ... Jimmy and Kiara made a big splash this summer when they were spotted on a date, resulting in mini-controversy in the football world. The QB later called it a "good learning experience" ... and we hadn't heard about it much since.