This manga, and animated adaptation, are often dismissed as cheap shock material. Its reputation proceeds it by such a margin that the story itself is left hidden in the dust. However, it is likely that what you know about this comic does not reflect much of the work itself. Suehiro Maruo is a complex man whos inspirations total things from grotesque woodprints of the Meiji and edo period, the upsetting reality of life in the early Showa era, as well as more contemporary works by Ero-Guro artists. This will likely put off a number off you, while those who are not likely read if were not introduced to these through Maruo. I do think that if one has any interest in the horror side of manga one should investigate this author.
The story is set during World War Two. A macabre reality for anyone not in the elites, especially for out protagonist who is a homeless child. This is a retelling of an older play about a flower selling girl who joins the circus, however told through the horrific vision of Maruo which dispelled any sort of romanticism one could possibly project. I do not know the politics of Maruo, I called him a complicated man as from interviews with him I couldn't gather a coherent image of his world view on politics or art. However I would like to bring up a short story be he penned that I read in Comics Underground Japan, another entry on this list, called Planet of the Japs. Its a retelling of the Pacific conflict during WW2 in which Japan wins, but just as the other stories in the collection it is overflowing with disgust at the fascistic ideology driving Japan during its Imperial years. He says through the story that Japan would have dropped the nukes on the US if they could, executed the leaders that would have controlled occupied Japan in the aftermath, and essentially done every cruel thing that America did. Japan's suffering isn't unique or noble but simply a result of imperialist expansion and the endless march of war machines. One could approach Shoujo Tsubaki with this in mind as even in the opening display of characters right wing politicians make an appearance never to appear in the story proper, it is simply the looming shadow of the ideologically driven misery that is present in the story.
The story itself is concerned with the young girl, a group of circus freaks, and an American magician who takes over the circus. It is easy to read into this as commentary about the American occupation of Japan, but for the sake of avoiding spoilers I'll let you do do this analysis yourself. I would like to focus on the freaks for a brief moment instead. At first everyone appears monstrous, impossibly grotesque and insanely cruel towards the protagonist. However, I don't think this is meant to demonise disability, but rather show an incredibly oppressed group in society who can only make money off of selling their own bodies as tools of performance on stage, and the way this marginalised group of misfits finds home. There is a trans character, someone born male who takes an appearance of a girl for her performance. Her identity is not ridiculed, and if anything the idea of having to abandon it to fit in with society framed with a deeply melancholic lens. Although, she is still cruel and malicious towards the protagonist. Could this be resentment build up over a lifetime of being ridiculed by people like her, a girl who until recently had the privilege of sleeping in her own bed. Is cruelty and violence simply a part of the natural order, even the plain looking people don't hesitate to hurt each other.