@Liquidxlax Keep in mind that mangaka have a team of assistants helping them. Did you know Toriyama (Dragon Ball) had someone else doing his backgrounds? That's really common. The only thing you can know for sure about a mangaka is they did the sketches for the story themselves, everything after that... could be an assistant. So One Punch Man has a team of people creating it, not just one guy. It looks great because it's got a TEAM of people with specialized skills all coming together to make it. That's how all professional manga are made, and American comics too (though it's more obvious with American comics because they list right up front who does pencils, inks, colors, etc.)
But most webtoons? One or two people, and on a much lower budget. Usually with a once a week schedule. You get amateurs to intermediates to professionals making these, so the quality varies wildly. The biggest factor will always be how much they're paid. Smaller budget, lower quality. I imagine the budget for this series is pretty low. I think the artist isn't that bad, but they need a dedicated background artist at least if they want to keep to their release schedule and still look nice. But it's unlikely they can afford it so... they're using cheats as a stop-gape.