Hey guys, basically the the title. Main lead where, the main char is acting dumb or hiding the fact the he/she is strong or smart? 

Few examples is maybe angry mom, dokgo rewind or pots of gold.  (One punch man :’), obv not anime :p)

  • +1 for Healer - ML pretends to be weak and timid
  • Mr. Sunshine - the FL pretends to be a weak and naive girl / noble who is sheltered from the world, but she is really a master sniper and secret agent for the Righteous Army; on a few occasions the ML also feigns ignorance or lets others assume he is less of a threat than he actually is
  • City Hunter - ML pretends to be weak
  • Pinocchio - ML pretends to be stupid
  • You Who Came from the Stars - an immortal extraterrestrial with super-strength, super-speed, and the ability to stop time pretends to be an ordinary professor
  • Strong Woman Do Bong Soon - the FL has super-strength but generally tries to keep it a secret and even acts weak in front of the boy she likes
  • The Masked Lover - the ML is an experienced cop who pretends to be an entry-level wannabe-gangster as part of a personal undercover mission and consequently moderates his personality and hides the extent of his skills / competence when he can
  • Where Stars Land - as a result of a major accident, the ML cannot move his right arm or leg on his own; he wears special metal devices that allow him to move normally, but they also give him super-strength, which he hides
  • Are You Human Too? - the ML is a robot with super-strength and advanced artificial intelligence but pretends to be a human
  • Sungkyunkwan Scandal - the SML pretends to be a dull-witted student who fails his classes and drinks and gambles in order to hide the fact that he is actually a vigilante rebel seeking justice + a master archer + very well read; the FL is very clever but hides her abilities / limits herself to transcription work and helping nobles cheat on the civil exam (until the ML intervenes) because she is a girl
  • Love in the Moonlight - my memory is a bit hazy on the details for this one, but I think the ML pretended to be weak and incompetent for reasons related to palace politics

Smaller instances:

  • Mad Dog - when the other characters first meet the ML, he pretends to be foolish in order to win a bet
  • Weightlifting Fairy Kim Bok Joo - the FL is a top university-level weightlifter but hides that fact when she develops a crush on a doctor who runs a weight loss clinic 
  • Story of Yanxi Palace - on several occasions the FL pretends to be weak or stupid to lure her opponents into a false sense of security or to carry out a scheme
  • The Eternal Love 2 - the ML loves teasing the FL by pretending to be injured and does so on multiple occasions

The underlined dramas are ones I rated 8 or above based on my personal enjoyment. 

The disguiser - well, he generally starts off as pampered young master, but he hides his skills behind that facade later on

Sparrow - ML acts like a brainless womanizer

Legend of Fu Yao - ML pretends for like 20 episodes (out of 66) that he is useless 

Thanks a lot folks! Will add them all to my list <3

This is not a drama but a great movie that I highly recommend (and applies to your description) is Secretly, Greatly

One of the funniest series I've seen is My Boss My Hero

It reminded me of Angry Mom, but less darker. The heir of a famous gang leader has to go back to school to earn the Yakuza Boss title.  He has to hide the fact that he's an adult criminal, but he's not that smart, and he learns more than he asked for.

✔ Lookout (pretends around bad guys) 

✔ Pinocchio (ML)

 Erica:

This is not a drama but a great movie that I highly recommend (and applies to your description) is Secretly, Greatly

Just finished watching it, was really good.