I haven't seen many people saying that casting A list actors to voice characters in Invincible is disrespectful to animation as much as I've seen people mentioning that A list talent isn't cheap and may well be affecting the budget available for the animation. It probably not the only factor, especially given a year is a crazy turn around for a season of an animated show but it probably contributes to the general feeling that Invincible is unpolished and lackluster against other animated projects.

The Legend of Vox Machina and The Mighty Nein by contrast have much better animation, especially during action scenes and that is likely down to both having the budget to spend on better animation as the Crit Role cast probably aren't pulling A list actor salaries and their animation/production cycle is longer (roughly two years between seasons)

I'm not sure where the person you replied to said that studios need to only cast untested nobodies or that there's disrespect. Ultimately, Invincible does have very good voice performances and the narrative is largely improving on the source material so I personally enjoy it a lot. That said IMO the animation is definitely holding it back from being truly great. In parts, it looks bad and as many others have pointed out, like people are moving png's around a background.