I do not think Ed Miliband is completely to blame for the result at all.
The Conservatives bashed Labour by blaming them for the financial crisis in Gordon Brown's term( especially the letter that said 'sorry we ran out of money' apparently.) That was something that was quite hard for Ed Miliband to argue against, but what he said was true; the Conservatives cannot talk about the future so they blame Labour for the past.
Ed Miliband was not the leader of the previous government that had ruined the Labour party's reputation and Conservative propaganda against the last Labour government which had an obvious detrimental effect on the Labour campaign which proved to be very difficult for Miliband to live down.
Also, I think people did not take Ed seriously enough because of his voice and his demeanour whilst eating bacon sandwiches, but I would not say that was all Miliband's fault(although he could have thought about his presentation in public a bit more.) People could not look past his slightly dorky, posh personality and look at his policies instead.
Also, Nicola Sturgeon ran a VERY strong campaign in Scotland and Labour needed Scotland to win, as many Scottish people are traditional Labour voters.
On the other hand, Labour's campaign was nowhere near as strong as the Conservatives, who used very strong techniques to turn people against Labour, Ed Miliband can be partly blamed for this. Cameron kept on emphasizing that Labour would ruin an economy that the Conservatives have built up for 5 years, and Miliband did not argue against this strongly enough, so everyone believed Cameron.
Also, I think he made a big mistake by ruling out a coalition with SNP. BIG MISTAKE.