Transmaxxing is a term from incel culture and it refers to the way som male-identifying incels decide to transition and live as women, as they think living is a woman is easier than living as a man. They also think that this will improve their love life and give them access to sex.
The underlying incel world view is that the incels are failed men who cannot live up to the masculine ideals of society and who therefore never will be happy or accepted as who they are. Nor will they be able to attract women. The term incel means "involuntary celibate".
Members of the incel community are often love-shy and introverted male assigned people who have a personality or a life experience that make them feel excluded from dating and courtship. This makes them extremely lonely and it is probably the despair that follows from the idea of never being able to have a good relationship with a woman that brings them to the incel community. In the incel community their deepest fears are confirmed, and they take comfort in the fact that they are not alone.
The incel philosophy is based on the most destructive parts of the hypermasculine narrative, where success in love is seen as the end result of a brutal struggle between masculine men and where women are reduced to more or less passive objects of these men's desire.
This extremely binary view of sex and gender makes it close to impossible for them to see that both gender identities and romantic and sexual preferences among both men and women vary immensely, and that there are most likely someone for everyone; the trick is to find them. It might also help to make some effort to make women like you.
Transmaxxing is based on the belief that the life of a woman is much easier than being a man. No doubt these male assigned people meet challenges many women do not, but the real life experience of women proves that it is not easier to be a woman. Indeed, women suffer from the same oppressive hypermasculine narrative as the incels, as they will have to live with the fear of male aggression and oppression all their lives.
The transmaxxing narrative is harming the whole transgender community, as it reinforces the transphobic idea that all trans women are "fake" or "traps". Ironically, most of the MTF incels who transition are most likely gender dysphoric trans women who have absorbed so much of the hypermasculine narrative that they are unable to see themselves as the women they are.
They make use of the only option this narrative gives them: That all women are like failed men and that they, as failed men, can find happiness in the role as the oppressed. This is a deeply misogynistic approach to sex and gender.
For a deeper discussion about transmaxxing, see this CDL discussion: Is transmaxxing a thing?