This is akin to the ‘Darth Jar Jar’ theory, where Jar Jar is theorised to be a powerful and clever sith lord who acts clumsy and stupid to hide in plain sight.
Preferably the term conveys a fake persona not just faked ignorance of an individual thing (someone who is openly intelligent but fakes ignorance of a single event doesn’t count for instance)
Please do comment other examples from fiction but with spoiler tags.
Such a person can be called a
Columbo
This comes from a US television program where the detective, Columbo, pretended to be well meaning but bumbling. Here's how you use it:
A helpful advocacy technique in special education meetings is to play Columbo and ask questions as though you didn't know the answer, for example, "Does IDEA allow the district to decide on its own to skip the triennial reevaluation?
Have you thought about using the Columbo approach?
Example I found on the internet:
Sometimes an attorney will pull a “Columbo” act and look like she is fumbling around trying to organize her notes or “thinking” of the next question when in reality, she is intentionally inserting a pregnant pause.
In the context of espionage, these terms would apply...
A sleeper agent is a spy who is placed in a target country or organization not to undertake an immediate mission but to act as a potential asset if activated.
Mole:
A spy (such as a double agent) who establishes a cover long before beginning espionage
