heat experiment--cotton

You can .
but why do you want to put it in the microwave oven?
It is a strange act.

But there is something wrong in your statement.
The cotton will not be heated because only water absorbs microwave and
produces heat.
If the cotton is wet, it is absolutely dangerous to do that.
If the cotton is dry, you can do it and there is no danger.
Note that when water absorbs microwave, water's temperature will rise a lot.