I guess as others have said...if they just do a poor job of it and I don't like it, I don't say anything.
If, however, it's very obviously some kind of mistake...like burnt to a crisp on one side, missing an ingredient, served totally cold, not cooked through, etc...I do say something. So me, if I had a restaurant and a literal mistake was coming out of the kitchen, I'd want to know. Like the rule of thumb is, it should be something that is quite obvious, not subjective. Everyone knows raw chicken is raw chicken and shouldn't be served on a plate LOL.
Historically, my experience has been that when you point that out to a manager or something, especially if you're gracious, they've both been very grateful to have it brought to their attention and eager to replace the meal, take it off the bill, and often both.
That has changed in the the last few years...they simply don't care at all. If they serve a drastically wrong meal (raw chicken, burnt meat, wrong ingredients, etc) and you bring it to their attention...they are just like "Oh, sorry to hear that." No offer to replace, no offer to take off the bill. My assumption is that given the tight margins and inflation and labor issues, they have seen a huge influx of customers complaining, both in earnest and disingenuously, and decided it's a matter of policy to not comp or replace anything no matter what.
I stopped saying anything, but it's crazy because sometimes a waitress will come get my plate, with one bite out of it, and be like "Oh didn't have the appetite you thought you did, huh?". Or "So, did you change your mind?" which I consider an invitation to explain. And I'll be like "no, I was hungry, but I ordered it medium rare, and as you can see it's dark brown throughout." And they're still like "Wow, it sure is! That sure isn't right!" and just drop the check LOL.