Found something on search, I will try to make an answer out of it.
https://okuha.com/how-much-does-a-mangaka-earn/
It’s actually not as common for a mangaka to be salaried, whether by
an animation company or a publisher. Manga is sold like books; how
much a mangaka earns is usually dependent on the contract they’re
offered.
[...]
That begs the question, what happens if the artist’s manga goes viral
and sells millions of copies? It’s not unheard of. Some mangas have
massive followings and seemingly exploded overnight.
Most manga authors receive a royalty from the sale of each volume, and
if a manga gets especially popular, the publisher is likely to
extend the contract for further volumes (and at a higher rate).
This is a tricky field for mangakas to navigate. While sudden success
may seem like a great thing, it also comes with a number of legal
loopholes and red tape to clear.
[...]
In particular, manga artists have to really advocate for themselves in
order to get the earnings that they deserve, especially when the manga
gets its own anime.
Some manga artists have remarked that allowed merchandise to be
produced of their characters in exchange for a licensing fee (which is
standard). But no royalties were ever negotiated.
Therefore, companies make hundreds of thousands of dollars from the
artists’ idea, while the artist only walks away with a much smaller
sum paid upfront. [...] Contracts are difficult to navigate, and
sometimes it’s best to get the advice of a lawyer before making any
major decisions.
From the way it's seems, it's all depends on the contract, so as to the question "does good art makes it better" seems kinda far, since we see less good drawn manga that's getting much more money only on being popular. The artist might claim to his publisher that his art is very high and all, but in the end it's depends on the publisher to agree, the artist needs the publisher more than the publisher needs him, being mangaka is a gamble - in the end it's the popular ones who gets more money on selling volumes than increasing art quality.
[My most claim is that it isn't mentioned in that article anything related about the art, at the end of things he says that even the popular ones doesn't get much, it's hard to say that a better drawn manga will bring his author more money, manga business is all about popularity after all. But the author might demands to get paid more because of that, but in the end it's all end up on the publisher decides, and in most cases it'll be - lets see if this manga will work out if any.]
