Yumeno is a pos.
Eh, she's in the middle of a deadline crunch, she's dealing with a block, she's frustrated, and she already didn't like Maekawa much. I think we're proably meeting her at her worst, and her worst really isn't that bad.
I would excuse her if this was the deadline, but in her inner dialogue she kept saying how she was the best of them all, and how she was annoyed that Maekawa won instead of her.
She's envious and jealous, but the author will redeem her too. There's not a single bad person in this manga.
I think you're misreading some of it? I think she was pretty much "redeemed" in this story. I mean, here:
https://dynasty-scans.com/chapters/still_sick_ch19#9
Leaving aside for now the "I'm a thoroughly worthless human being except for my manga", (she's definitely not a egotistical person overall, this is just the only thing she's got)
She's more upset at the way Maekawa downplayed the effort involved in winning? She's submitted repeatedly and kept striving to improve, and then here comes Maekawa with "Gee whiz I heard there was this contest thing so I figured I'd give it a shot." It's about first impressions. Maekawa came across as making light of something she'd worked really hard and strove for.
Then she only gets her shot because Maekawa (from her perspective) flaked out. That's not gonna make her look any better, and for somebody with already low self esteem, it's gotta be a bit of a bitter pill.
Later we get this:
https://dynasty-scans.com/chapters/still_sick_ch19#19
This is the next time we get her internal monologue, and, yeah, she starts with "you just got lucky, I'm the real talent", but "That was what I believed" makes it pretty clear she's admitting that she was -wrong-, and that's the turning point at which she changes the way she treats Maekawa for the rest of the story.