He was a bit too honest. You saw in the Denmark/Croatia game the goalkeepers standing a yard away from the takers, bellowing at them before they took it, and then launching three steps off their line each time. Akinfeev did the latter each time as well. De Gea did neither.
I don't really care about his performances for Spain, but he could probably do with adding a bit of cynicism and nastiness to his game in general. I don't think his career and his reputation is going to rest on how well he does at penalties, as he is still top class at just about everything else, but seeing every goalkeeper bending the rules quite heavily, then he should be doing the same.
Overall, I'm not worried. The calls of him not being that good anymore are a bit daft. He's still the very best at open play saves, close range, one on ones etc, and that's a much more significant part of goalkeeping and what sets him apart. There's nobody better in that respect. Kasper Schmeichel is very good at penalties, but never in a million years would I want to swap him for De Gea.
Messi is crap at taking penalties, but you wouldn't swap him for Rickie Lambert.