I have lost track of how much Mami does or does not know about the witchbomb. Is it:
"There's something that would be dangerous to know"
"There's something that would be dangerous to know, about being a magical girl"
"There's something that would be dangerous to know, about what happens when a soul gem fills up"
"There's something that would be dangerous to know, about Witches"
Or any of those but with "dangerous" replaced by "distressing", i.e. dangerous because of Mami's fragility and the risk of spiral, not dangerous in and of itself by some magical knowledge-based effect, or by "extremely secret", because someone (probably Kyubey) would try to kill her if they knew she knew.
Some of those possibilities affect the chance that she might figure it out herself - good odds she already suspects.
The last affects her odds of potentially deciding she wants to know; if she thinks it's inherently hazardous, she'd almost certainly wait for us to say it's safe. If she thinks it's her own mental state making it dangerous, she might think she's ready before she actually is. If she thinks it's a secrecy-risk... Unrelated suspicious occurrences might get her wondering on entirely the wrong path.