Agreed. Generic boxarts are a dime a dozen ... but the reason why it's especially bad for Doom is that Beth/id's been trying to market the game as a return to form. That's not to say it needs to have the exact same aesthetic, but at the very least it needs to look and feel inspired. They're definitely trying to get away from the reputation that Doom 3 left (a much more generic shooting experience).
Right, it is just box art, and at the end of the day it doesn't matter a whiff what's on the box as long as what's in the box is awesome...
But great games tend to get all the details right, up and down the line. When the box is bad, you wonder what else the producer didn't care about.
(*There are exceptions, of course; Ico and Mega Man for instance have boxes so bad they've required fans to campaign to get the word out about the quality of the actual game. Do note though that in both of those cases, it was the outside publisher putting its interpretation of branding on the product once handed something they needed to market, whereas nowadays the keyart is drafted as part of the production and sometimes even comes from the in-house design team; the guys who make the game are a part of deciding what image will represent their game.)
Mostly, who cares. But after following the travails of Doom 4 for 8 years now, it sadly did not surprise me to see the game going to market with a cover so generic and lifeless and tired. Every single indicator big or small has whispered, "Don't get your hopes up."
Substance trumps style, but style can reflect substance.