I think Martz was a fine coach but one who was set up to fail. He made his share of bad decisions but I don't think he should be expected to not go through growing pains as a first time head coach. What I don't know, and where the initial problem that everything eventually fell on, was the management and support structure the Rams provided him. He didn't have a good management team, didn't have a good scouting department, didn't have good ownership and the coaching depart quickly eroded or was pilfered.
Martz wasn't blameless by any means. He royally screwed up the Warner injuries and benching. Hubris probably played a large role in his questionable decisions. But those were forgivable IMO. He was in over his head and had little real help much of the time. On top of that, he had opposition from his own side.
Give Martz the same opportunities and support that McVay got and I think he would have been even more successful than he was.