Mao Marimo does a few things different in comparison to your average gender bender manga. Unfortunately, this does not save it from being entirely mediocre.
The Good: Instead of the series being a comedy about our main character Mao failing to adjust to femininity, she takes to it right away and decides that she is fine with being a girl if that is what fate has in store for her. Most everyone else takes positively to the change, too - with the exception of Mao's childhood friend Jiu, and Mao's twin sister Ako, who both initially want "her" to be a "him" again. This sort of
reversed dynamic from the average gender bender, where main character's friends are struggling to change the main character back but the main character themselves accepts their new gender without any angst or any protesting that she's "really a boy", is a bit refreshing. What's also refreshing is the serious way the cause of Mao's transformation is portrayed, the guardian spirit of their village's lake who is as intimidating and otherworldly as you'd expect a great spirit to be.
The Bad: Despite the author trying to mostly keep a serious tone to the manga, all that the series really has to offer is Mao being thrown into ecchi situation after ecchi situation, each time having Mao acting more oblivious and suggestible than the last. This might have been alright if the art was half-decent or the series was actually trying to be a comedy, but neither is the case. Aesthetically the series is pretty sub par, and the only plot development comes in the form of generic love triangles that the author tries and fails to present in a serious tone - Mao falling for an older man that has no interest in her, Jiu falling for his childhood best friend "as a girl", and Ako falling for her twin brother "as a boy". None of these relationships are handled remotely interestingly, all that ever happens is the usual misunderstandings, blushing, and the occasional tsundere moment. And Mao herself is infuriatingly devoid of any real characterization or development. She seems to just accept whatever everyone else in the story wants, even when they're acting like complete idiots or jerks.
All in all, I won't go so far to call it a bad manga, but it's probably not worth your time.