While her first appearance in Star Wars films came in The Force Awakens, which is set after the time period the show is said to take place during, the inclusion of Maz Kanata in The Mandalorian would not be out-of-place.
Maz is said to have lived for several centuries and Han Solo said that she ran the cantina on the planet Takodana for more that a 1,000 years, meaning that she would have been around well before this time period and was running her bar during it.
Maz herself is a former pirate and from what we saw in The Force Awakens, there were plenty of shady characters upon the arrival of Han Solo, who himself knew her from his career as a smuggler.
Being that the protagonist of The Mandalorian is said to be a lone gunman and operates away from the involvement of government bodies, including both the First Order and the Republic, a character such as himself stopping by Maz’s cantina, which is said to be a safe-haven for criminals, sounds like it could fit well into the story.
I could very easily see the protagonist stop by her cantina to lay low from any pursuers, find transportation, or even get some work leads from Maz herself.
Additionally, Maz was a very interesting character from the new trilogy and personally, I was disappointed that aside from a brief appearance on a hologram in The Last Jedi, she wasn’t really featured in the last movie.
The inclusion of her would be a big benefit to The Mandalorian, and as she is from the newer movies, would help to further the link between the new show and the films.