For me this isnt really about criticism but about encourging groups to improve their scanlations.
Adding penalties to that just does not seem like a good idea to me.
You can also criticise them already by opening a thread for a chapter or group and tell them what you are dissatisfied with.
We cant force people to improve, its up to them if they want to or not.
But we could tell them: Hey look, people like what you did, here have some cake.
Or we could tell them: Hey look, here is your social credit score, have fun.
Both will have likely some kind of impact, personally I always prefer the positive one over the negative one when it comes to encouragement.
Thinking about it further. What can we do in actively helping them improve?
Readers probably arent even the correct group of people to get feedback from. I would assume they are very unlikely to provide valuable feedback at all, maybe minimal. I guess the best feedback you can get is by other scanlators.
As an example, as a developer I wouldnt get my feedback regarding the code I have written by users of the application but instead by other developers.
I could get feedback on things like if a feature is well implemented, if it got bugs, if it has the expected behavior by users but those wont improve me as a developer but would improve the application in general.
So with that analogy, if a scanlator wants to improve the behind the scenes of their scanlation they should get feedback from their peers.
If mangadex wanted to go for something like that, it would need to implement a feature to connect groups with each other and have something like peer reviewing of scanlations.
Maybe something like github or gitlab. In which mangas would be open repositories which belong to a group, chapters / scanlations could be maybe seen as branches on which commites / changes happen, which could be reviewed by others.
Of course this more like a very abstract idea and I am kinda doubtful if groups would even want something like that in anyway.
The other unanswered thing is if the majority of scanlators even want encouragement to improve.
Because this isnt really something for readers but for scanlators in my opinion.