It’s a word I would use. They look cheap. The white background with the cartoony design looks like something my son would see in one of his “how to draw manga” books.
As an artist myself, and having worked with professional artists for the past 20 years, I have to take some issue with this.
First, Manga is just a different style, it's not any less skilled or "cheap" than any other art.
Secondly, let's say for the sake of argument that manga is is faster to paint/draw. Let's compare a manga piece with the cover and look at the differences. The covers of these books contain many more layers than the manga piece does. There's hue, saturation, shading, and highlights for every color used in these covers, and there are many more colors compared to manga. The pencils and lineart are much more detailed. The amount of time spent on one of the cover images compared to above manga image is exponentially greater. That's not "cheap" art.
Quite frankly, anyone who thinks this art is comparable to basic manga and is cheap doesn't understand anything about art. You (general you) can not like it, and that's perfectly fine. But making value judgments about something you don't understand is different.
But again, that assumes that Manga is less skilled or time consuming than any other type of art, which it's not. I used a very basic Manga piece up there for comparison, but if you look at much Manga, it has all of those things I mentioned that makes a piece of art complex and time consuming (dozens of layers, etc). Take this for example. It also includes shading, hue, highlights, lineart, etc. Bottom line, Manga is just a style, it's not any more "cheap" than any other form of art. Whether or not we like a particular art is entirely subjective. How much work and talent goes into a particular art style is not.