A cameo appearance by the Monitor in Legends of Tomorrow's season 4 finale highlights how the black sheep show best captures the spirit of the DC Comics' universe out of all of the Arrowverse series. This, in turn, makes Legends of Tomorrow the strongest of the four Arrowverse shows, due to how well it balances both the silly and the serious aspects of the source material.
First appearing in the 2018 Arrowverse crossover event Elseworlds, the Monitor is a mysterious cosmic being who has been testing the heroes of various realities for some time. He explained that his ultimate goal was to find an army capable of averting a coming crisis that threatens all of the multiverse. The Monitor has since appeared in the season finales of both Arrow and Supergirl, transporting the future version of Felicity Smoak and freeing the Martian Manhunter's brother from a form of imprisonment, respectively.
These cameos established the Monitor's power and the idea that it is a very big deal when he deigns to appear to lesser beings. As such, the audience feels an immediate tension when the Monitor is seen in the crowd at the Heroes Vs. Monsters stage show in Legends of Tomorrow's season 4 finale "Hey, World!" Because of the Monitor's earlier appearances, the expectation is that something big and bad is about to happen. Sure enough, a wicked witch crashes the show with a dragon in tow, but when the camera cuts back to the Monitor, he is simply eating popcorn, without making any dire proclamations or indeed speaking to the Legends at all.
The secret of all great comedy is surprise and working against the audience's expectations. With that idea in mind, Legends of Tomorrow was reinvented as a more comedic superhero show to make it stand apart from the dour Arrow and the more traditional The Flash. Paradoxically, it did this while tackling some subject matter that sounds serious on paper and introducing characters from the comics who are not known for their humor.
Legends of Tomorrow's use of the warlock John Constantine is another prime example of this balancing act in action. While the classic Hellblazer comics are full of dark humor, Constantine himself is most often depicted in a serious light. This made actor Matt Ryan, who plays a picture-perfect John Constantine, into a perfect straight man for some of the other characters more overtly comedic shenanigans. It also added to the humor when Constantine was forced to play against his image, such as when John was press-ganged into becoming a summer camp counselor.
Another example of this is how Legends of Tomorrow season 4 featured no less than three subplots involving heroes who had to save their beloveds from Hell. While this was played straight with John Constantine's efforts to save his boyfriend Desmond's soul, it was played for laughs when Sara Lance had to save Ava Sharpe from a personal purgatory that resembled an Ikea outlet. The finale also saw sorceress Nora Darhk having to search Hell for her lost boyfriend Ray Palmer while dressed as a fairy godmother. Ridiculous? Yes, but still heroic at heart and that is the spirit of DC Comics in a nutshell.