@Lilliwyt
you missed the context
Barring any complication, key visual/promo art should be the main designer job and on peco they dismiss that in favor of boosting sales.
Aoba was gung-ho about doing it before it's given to yagami, hence her disappointment eventhough in the end she admit that yagami's work was better than her.
if you re-read the past chapter where the job given to yagami (chapter 49), she was also adamant against it as it should be the main designer who do the key visual. but they keep pushing it to the point that if yagami didn't do it then they'll pay another famous illustrator.
So what hazuki means by 'being sincere in production' isn't about choosing unknown name instead of using somebody's fame to boost sales, but letting the main designer do her job regardless of her fame.
If aoba is the main designer this time, they'll let her do the promo art regardless the fact that she's more well known now.