There's nothing wrong in microwaving water to heat it up for making tea with.
I've been microwaving water for making a cup of tea for many years and it's never been a problem.
You could also just run the hot water tap from your kitchen faucet to get hot water for the tea but it's faster to just fill a cup with water and then pop it in the microwave for a minute or two and then add the tea bag to the hot water.
The microwave makes the water hot enough to where it only takes around 4 to 5 minutes for the tea to brew from the tea bag.
The microwave won't cause you any harm when heating up water just like it won't cause you harm when cooking food.
Although microwaving food or water, coffee etc is not the healthiest it's still not gonna harm your health.