In fact the whole reason she goes back to 1910 is to try and stop Kang from changing the outcome of WW1.

Only after she joins the TVA. She would have been a major player in Loke season 3 but they had to scrap that story line when they fired Jonathan Majors as Kang.

Star Lord even references Mary Poppins in GOTG Volume 2.

Off screen Maria is exposed to the radiation of the tesseract after being captured by Nazis/hydra. As a result she develops telekinetic powers and eternal youth. The British eventually recruit her to fight in the war under the code name Poppins.

Both films are Disney properties now which means Mary Poppins is actually an MCU sequel to Sound of Music.

"La, bla bla bla bla bla bla!"