Without the personality, it doesn’t matter how good the backstory is. They’re both important, don’t get me wrong, and I’ve said as such. But it doesn’t matter if you get the backstory right and get the character wrong.
Now, in rare cases, you get characters that can vary widely. This typically comes with a character that has existed for a long period of time. Batman and Bond being prime examples. However, the personality differences must be consistent with the world they’re placed in. There’s a reason why people didn’t enjoy BF and B&R after B89 and Batman Returns. The world was set up as a darker interpretation. The personality and tonal change therefore felt out of place.
Long story short, the backstory is important as it informs the personality of the character, but without the personality, you have nothing. You have no character. People don’t fall in love with Peter Parker because his uncle died, they fall in love with him because of his choices, his humor, his drive.
Which is why, I’ll say again, I will never agree that backstory is the most important aspect of a character. And to that point, I don’t agree that Raimi’s Mary Jane represented the key MJ character traits. She just wasn’t very well developed, which is a bummer. I wish she had been, but the writing they gave her was rough.