I'll take a guess and say that "being nice" in this context means "being courteous, kind and respectful towards women" something that women claim to like in a generic and vague manner.
But everybody knows that alone isn't enough.
What does being “courteous, kind and respectful” towards someone mean to you
specifically. There’s a lot of people out there who describe themselves that way (Chris Chan, most infamously) who are anything but.
I’ve met many people who claimed that they were “nice”, but what “nice” meant to them was “if I don’t call this person a bitch, maybe she’ll fuck me”. Desperation can be smelled from a mile away, by anyone with any social awareness. Not just women.
Taking a shower or changing clothes ain't gonna make a loony less of a loony. If you're talking about seeing a shrink, well, therapy is something that can go on for one's entire life. Some mental damages are harder to fix than others.
Seeing a shrink and practicing good hygiene is a lot better than not doing so. When you’re unhygienic, you’re actually going to make your own mental health a lot worse than it has to be. Keeping your home organized is one small thing that helps with building confidence in oneself.
It’s why therapists always suggest cleaning your home and yourself as a first, crucial step to self improvement.
Incels have zero confidence, because they give themselves nothing to be confident about outside of vague ideas as to what “nice” means.
Wallowing in self pity is a defining characteristic of almost all incels, whenever they don’t have inflated grandiose ideas of themselves. Both traits are repulsive to just about everyone, not just women.
Women as a whole are a lot quicker to pick up on those things as red flags, because of how women grew up being socialized.
Which in turns boils down to the fact that women are the selectors. Humans are social animals, which means socialization comes with rules. Those who cannot follow social rules are doomed to never contribute to the gene pool. Those who refuse to learn and choose to isolate themselves would have never survived past 15 in ye olden days.