What exactly do you mean by mob character?
To me, a mob character is a background character. Even if you go with an army setting, where all the background characters a big, beefy Spartan warriors winning battle after battle, the actual story will still be about the non-background characters, which means that the main characters' importance is not diminished no matter how competent or useful the mob characters are.

you make it like dead parade or DRRRR and spam as many superficial characters as possible

It doesn't matter what they do when they're just a featureless blob of copies. Everything about their generic, doll-like appearance tells us not to value them at all since the universe itself says they're not important or useful. So, the first step would be to humanize them by giving them identifiable visual traits, but that's a whole lot of work.
I think Konosuba is noteworthy in this way. It still has faceless knights and such, but the guild often features individuals instead of copy-paste nonsense. Because of this, events like pic-related make it seem much more meaningful.

>>126670
But in settings like Kimetsu or Kaijuu 8 the setting tells us that all of these rank-and-file soldiers are skilled enough to go through the same trials the main characters did and have more experience than them, at least at first. Designs are a meta factor for helping the audience know who they should remember, they imply relevance to the story, not competency.

shonen series are not good stories, they're good explorations in character growth, but never worldbuilding and plot

Typically, the MC has some special divine ability that separates him from everyone else in-world. So you can make your background characters strong in the usual human ways while only making them appear weak to the MC and whomever the MC is fighting at that time.

It isn't hard to write strong background characters. But most people skip on the world building now. Since it takes up far more runtime and words to flesh out background character details.

I do appreciate studios that don't copy/paste background characters and at least put some effort into them. The first couple of shows that started not to bother and use blobs/cut outs were funny. But it got much less funny as time went on.

Lately I've been trying to spot a male background character in MahoAko thinking they might have hidden one somewhere as a joke. I thought I found a couple. But on closer inspection all of them turned out to be lesbians.

>>126637
Avoiding the Redshirt concept (point 1) and screentime (point 2).
The Redshirt concept is that the characters are disposable and are there just to die. Not to say you can't have them die, but give their purpose something more before dying. Their actions and subsequent deaths push the plot along, maybe they sacrificed themselves to buy the heros time for example. Or for a more in depth example: a group of knights storm the walls while the main characters are sneaking in through tunnels. Many of them die but they succeed and the result has a knock on effect that aids the main characters, then at the end we see what remains of the mobs they had a brief interaction with earlier. While they were never the focus and we only see the before and after, it still gives us a glimpse of a wider world even if we were focused on the main characters.

Mobs are either dressing for a scene or at best they're minor characters that show up once or twice that interact with the main characters, but that's not to say they're unimportant. Their job is to do things the main characters cant do and support/hinder them in the grand scheme of things. The main characters cant exist in a vacuum and thus need mobs to hold up their share of the story, the background cast and extras exist to give contrast to the main characters and give the world some semblance of life and potential stakes to care about.

As for importance, I assume you mean to the story. The actual power of the main characters and mobs are irrelevant, what matters is how their interactions and screentime affect the story told around the main characters.