Too Much Dialogue in MG?

Try action instead of description.

But good dialogue never bores.

David, I think maybe you're not reading enough to balance out your writing...? Because that's the kind of thing you can look for in the books that you enjoy. Grab a handful of really, truly successful books that are most like what you're trying to write, and do some analyzing. You could even copy a few pages and mark them up with colored highlighters: Blue for dialogue, yellow for action, pink for description, green for internal stuff like ruminating and remembering. You'll definitely see the balance and the rhythm that way.

Basically, too much of anything isn't good. The more you can weave them all together, the more the reader can lose herself in the story. And different parts of the story will need different mixes. The first time a character enters a setting, you need more description so we can see it in our heads. After that, you only have to show what's changed, or mention it as the character interacts with things in that setting, or describe it if the mood needs to be strengthened.

A fight scene would need lots of action, obviously, but with short bursts of dialogue. A teary conversation between two characters would be mostly dialogue, but need some action to keep it flowing and real. It's all about finding the right pacing for that particular moment.

And then there's voice and style. Some authors are wordy, some are lean and to the point. If your descriptions are spot-on and vivid, they can be longer than if they're just informational. If you're really good at wise-cracking conversations, then you don't want to interrupt the flow with a bunch of details. If you're writing Gothic fantasy, you need a lot more description than if you're writing contemporary humor.

So... can't possibly answer your question!

Every book is different, and a great many stranger things than a lot of dialogue have made for successful (and unsuccessful) books. There's no way I can tell you from here if you've got too much dialogue. The best thing I can recommend if you're worried is to share a couple of chapters with someone you trust (or SYW,) or to wait a week or so and re-read it yourself and see how you feel about it.

In order to keep my writing fresh and fast-paced, a lot of my description has been cut. Sure, this seems to improve the flow... But it also seems to be making the story 90% dialogue.

Is this good or bad?

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I seriously doubt your story has that high a percentage of dialogue, but if your gut is telling you there's an imbalance, trust it.

I don't think you need to sacrifice description to keep the pace moving, just weave it into the action. Sarah Prineas' THE MAGIC THIEF is a great example of maintaining forward momentum while firmly grounding the reader in the surroundings. If you haven't read it, I highly recommend taking the time to do so.