Defrosting anything in the microwave is dodgy. They're designed for serious heating, and don't heat completely evenly, so they tend to start cooking food before it's fully defrosted, even on low power. It might not be quite as bad as "defrosting" in a pan on the stove, but it's not great either.
That's especially bad for fish, since it cooks and overcooks quickly. You can maybe get away with a quick partial thaw in the microwave if you're careful, but it wouldn't be my first choice - it's pretty risky in terms of quality.
If you're in a hurry, I'd definitely suggest thawing with running water, as described for example in this question/answer. Of the safe methods, it's the fastest one that won't also cook your food. Especially for small/thin things like filets, it's pretty fast, and on top of that you can err on the side of not fully thawing to save time, and let it finish thawing while it cooks.