I haven't played the computer game, but ... you do roll three dice instead of one when you're in cover. Cover is three times as good as no cover in preventing damage. Are you sure that still classifies as "not good enough"?
Also, if you want to make cover stronger, then you will probably also have to make Locust attacks stronger. Because the game has been balanced with exactly the values that are in the rules. If you want to make your game too easy, feel free, it's your game after all and you can do with it whatever you want - just rest assured that if you win with your modified rules and stronger cover without giving the Locust stronger attacks to compensate, you might not have won the real game
The chances of rolling three blanks when in full cover are still only 12.5%. It's not common, but one out of every eight attacks can be great shots. The dice are set up the way they are specifically to give an advantage to the attackers. With guaranteed shields, it turns into a stalemate.
I already don't attack Locusts when they have full cover, because it's usually a wasted attack, so bumping up their defense when in cover doesn't make them any stronger. Since they will still attack you while you're in full cover, the net effect is making the game easier for the players.
And like the other guy said, if that's what you want, you go right ahead. I don't have any issue thematically with the idea that you can still be shot when you're behind cover. There are always body parts sticking out of places in the videogame, after all.