
A unique, flavorful, and annoying weakness. Looking at the two elements, the effect and the discard condition, we get:
The effect: only goes off during a search, cancels the search entirely, and you draw an encounter card. Another very situational card. It can wreck your plans or kind of fizzle. I mean, you could build a Mandy deck with no search effects beyond her . You'd be abandoning her ability and signature asset, but you'd also neutralize her signature weakness, making her kind of a bland "generic seeker." That's not a good idea, mind you, but I am not sure there is another investigator who can render her signatures irrelevant, so that's one mitigation strategy....
More practically, you want to find a big search card to draw this out. Like thakaris, I think Research Librarian is probably your best choice to "rip off the band-aid" and get this out of the way early. (I'm also very impressed by their subversion of Treacheries, but, Arkham being Arkham, you would probably draw one of them as your draw from Shocking Discovery instead of before it.) Another strategy is to use Whitton Greene to dig for, say, Segment of Onyx. Since her search ability is limited by revealing locations, it hurts less than searches driven by card play or secrets. Cards like No Stone Unturned (5) are pretty expensive to spend on a wasted action and an encounter card. Depending on your "off class," Mandy has some ability to cancel or redirect the errant encounter card as well.
The discard condition: It discards itself.
All in all, this is a below average signature weakness, maybe even way below.