But standing there posturing and sending men to fight in his stead is perfectly in character. No one would have been asking him to lead the charge, just not go inside just yet. And actually, Joffrey is exactly arrogant enough to think he can charge into a fight and sweep the foe before him. He wouldn't REALLY wet his pants and run crying to mommy until they started to fight back.

They erred by trying to make Cersei too human early on, and to continue that notion this year, they made Joffrey more of a free-willed character, rather than a playing piece being moved by Cersei or Tyrion who fought for control of him. That whole crisis was not about a failure of leadership on Joffrey's part, but on Cersei's, because she was being so short-sighted in wasting the defensive efforts merely to preserve Joffrey's safety, even Lancel started to become disillusioned with her.

And that too ties in with the contrasts between Cersei's style & Sansa's. For all that she seems stupid, Sansa is right, and more effective than Cersei & Joffrey in some ways. Meanwhile, you have Cersei ridiculing and berating Sansa for being soft and weak, but Mommy Dearest is too soft to endanger her precious baby (and in the book it was a much more relative danger, with him in good armor, on the walls, with top guards, well away from any direct combat and no one asking him to get into the fray), and in the process, loses the fear of her subjects she prizes above their love. Once they think the battle is lost & Cersei doomed, what is there to be afraid of? If the brave lioness Cersei is afraid for the King's safety, maybe everyone should be afraid, and should grab what they can and escape while they can.

This is all part of an ongoing theme where Martin SHOWS how all that pragmatic brutality is bullshit, even while he has the characters TELL us how effective or necessary it is. Cersei's terror was not enough to keep the loyalty, but Sansa's compassion saves her from the fate Cersei forewarned.