While it's a popular acronym, it's important to remember that the full phrase is considered vulgar and may not be appropriate for all contexts.
It's unclear what Bella Thorne's specific usage of this acronym might be. However, it's likely that she is using it as an expression of amusement or humor, as the acronym is commonly used to express that someone is finding something very funny.
If you're unsure whether it's appropriate to use this acronym, it's always best to err on the side of caution and choose a more polite alternative like "LOL" or "haha."