Mayumi's main characteristic is her free-spirited nature, which results in her being a liar and a cheater. That she's bisexual is entirely irrelevant to that fact. In fact, it's only mentioned in once over the span of two lines.
Fujiko, however, is also clearly bisexual. Her dedicated love for Mayumi is covered over the course of multiple pages. The same can be said of her love and dedication for her deceased husband.
So I'd argue that the depiction of bisexuals in this particular yuri manga is fine. Some people lie and cheat. Some people don't.
But of course everyone cheat, being straight, bi, pan, gay, lesbian or whatever.
But, in case of Yuri in general, most of the time the one who cheat happen to be bisexual. And in my message, I'm not only talking only about cheating but the character of the bisexual in general.
I'm mean every bisexual are not sex-crazy, open minded in relationships, extravert, partying everytime, insensible, drunkard person in real life. But that's stereotype we mostly found in Yuri. And even I would say in a lot of Media (Mangas, anime, series, movies, books, etc ...)
In the case of this story, the only who actually state her sexual orientation is Miss Red Lipstick. The MC could be straight with one exception for this woman, She never stated her sexual orientation and never loved or go out with another woman besides her.
For me, there's only one actual bisexual. And for the MC, we don't know, maybe she's straight with lesbian tendencies for specific women? or bi but she never said it.
And I would say, most of the protagonist in Yuri are like the MC. Most of the time, Authors dont stated their sexual orientation. They just love another girl/woman and that's it.