Middle-grade targeted or not, I think backstory should be exposed gradually.
I can remember kids (and not-so-kids) complain about a long piece of text without dialogue. At some age of one's reading career, one realizes that's where many interesting things happen (or else that one doesn't like reading), and then complains about too long descriptions and information dumps. I won't enter in discussing if that's the way of getting old or getting used to boring stuff. What is sure is that kids don't want boring stuff.
Therefore, the question you should ask yourself, as What said in his comment above, is if your prologue is boring. I have a prologue that starts with dialogue and continues with blood spilling, swords swinging and stuff blowing up, so I don't believe a prologue means info dump.
I think the best is to hook the readers with questions, not answers. Is all your backstory absolutely essential to understand anything of what is going on? Maybe you can leave some stuff to be explained later. You can also try to identify the one thing that matters the most as backstory, and then explain it with a scene (some action happening that explains the backstory) that will conform the prologue.
Answering the question directly, it can be good (look at Rowling's HP and The Philosopher's Stone), and it can be bad as in any other book. I think the middle grade fiction demands higher quality standards at least, not lesser, than other targets; because middle graders get bored more easily, they are new in reading, they have to be captivated to keep them, and they have a lot more things to do.