To check Baby Groot breaking the fourth wall in Guardians of the Galaxy Vol. 2 at minute 2:41 mark in the video below.
In addition to being the first movie to utilize Red’s 8K camera, Guardians of the Galaxy Vol. 2 was also the first movie from within the MCU to break the fourth wall.
The breaking of the fourth wall is a shift in reality in a movie when a character acknowledges the audience.
Though Deadpool is in the Marvel comics universe, he is not involved in the MCU, so Guardians of the Galaxy Vol. 2 is officially the first Marvel Cinematic Universe movie to break that fourth wall in a very sly way.