There are very good reasons not to microwave tea, first of all:

• I usually find microwaving anything in a mug adds some unpleasant flavour from the mug. Using glass eliminates this, but worth noting.

• Microwaving the tea itself will break down some compounds and release more tannins, your tea will be worse.

But even if you're just microwaving the water, the kettle wins (depending on what tea you are brewing). Black tea should be brewed as close as possible to 100°C - when you have a kettle you should pour it just as it comes off the boil, around 90-95°C. By that point the water has actually been boiling for quite a while (at least the water around the element), allowing the rest of it to heat up. It's very difficult to achieve this in a microwave, and dangerous too since you can just end up spraying boiling water around your microwave.