If recent events (in particular, Zapata's goal for Atalanta) where a one off or a rarity, then I'd totally agree with you that labelling De Gea as cowardly would be harsh.
However, when the same thing happens time and time again, (at big moments that cost us such as Kluiverts goal for Leipzig that knocked us out of the champions League last season) then I think the fans have every right to ask some serious questions.
Yes, he's a great shot stopper, but the inescapable fact is that there is more to goal-keeping than shot-stopping and being brave and putting your body on the line is a trait that all the best goal-keepers should have, De Gea simply doesn't have that trait and that's just not good enough, especially when you consider the wages he's on.
When it comes to 50/50 balls or incidents where he may have to, as I said, be brave and put his body on the line, more often than not he totally bottles it and, unbelievably, almost seems to want to get out of the way.
As I said l, there are numerous incidents, dating back a long time, one that really sticks out in my mind was against Wolves a few seasons ago. It was a huge game where a win would have put us on the top 4, Smalling scored an own goal, but, as you can see from the link below, I honestly think most top keepers would have simply came out, been brave and smothered the ball. De Gea, as he often does, seemed to do nothing except turn away from the ball, fearful of getting hurt.
http://www.soccer-blogger.com/2019/...-vs-wolves-2019-bizarre-smalling-handball-og/
What makes matters worse is the fact that, this doesn't really get registered as a goal-keeping error, I remember on the night there was no mention about what De Gea could have done to prevent the goal, Smalling was the one getting all the blame (yes, it was his fault too but as I said, most keepers would have bailed him out with a simple bit of awareness and bravery)
This is the problem, there's no accountability for him when he makes these sort of errors because people get blinded by the fact that he's a good shot stopper (as most keepers are anyway).