The thing is, GOG is the distributor not the publisher nor developer of the games they sell so they don't create the art for the games, they are given the artwork from the publisher to use and in some cases they may be given a set of artwork to use to hack up something perhaps. In lieu of that they might scan in original materials they've been sent by the publisher, or buy stuff off of Ebay to use with proper permission, etc.
Chances are extremely high that if the artwork for a game sucks, that that is all they've been given by the publisher and have at their disposal. You have to keep in mind that a lot of old games were not preserved well over time and even original publishers no longer have their original artwork or even the source code for their games. This is why some of the artwork, manuals or other content are scans and some of them might be low quality because the publisher didn't do a good job of scanning it, or perhaps the scan actually came from an end user who owned the original game and scanned it or took a digital photo who is not an expert graphic artist or photographer and then submitted it and having something is better than nothing.
I would much rather GOG ship a low quality scan of a manual or artwork than ship nothing at all if they don't have anything better that they can offer. Something is better than nothing. And I give them the benefit of doubt that they search hard for this stuff and try to give us the best that they can offer because they have a history of doing that, and a history of updating things if and when better material or new material becomes available to them or they're able to track it down etc.
If someone is going to be disappointed by low quality scans that is ok I think, but ultimately it should be directed at the owner of the property - the game publisher, and not at GOG. GOG is a store, even if they do sometimes whip up some artwork with the proper permissions, they're the store, not the content creator. So discontent really should be directed at the publisher, not GOG.
Even then though, what if the publisher has nothing more than an old boxed copy of the game themselves? The publisher might have acquired the game through an acquisition that was an acquisition of an acquisition of an acquisition from 25 years ago. If the publisher does not have anything better and is unable to track down anything better, would people rather have a game as-is best effort, or not at all unless it looks like it came out yesterday quality of artwork?
I'd rather have the best they have to offer even if it is "crap" quality because that's better than not having a game be available at all. Some of these games are old enough that it's kind of like going to a museum of ancient Egyptian artifacts and then complaining that the hieroglyphics are faded and parts are missing from erosion and that one wants a refund because the museum should be able to do better and offer ancient art that looks like it was made yesterday. :)
Having said that, if someone has or knows where GOG can obtain a better manual, artwork or other materials for _any_ game in the catalogue which is better than what they currently have, send them a message and tell them about it, and if you have an old manual and can scan it high quality, or maps or whatever, offer that to them too. They may have to get permission from the publisher to use the content but that doesn't seem to be a problem in the past, and I'm sure both GOG, the userbase and even the publisher might appreciate such efforts as they really are not far off from my egyptian antiquities analogy in terms of gaming history in many cases. :)