In MetaMask, you could simply copy your address by clicking on it.
When you hover over the address, a tag “Copy to clipboard” would appear.
New User Interface (UI)
In the new UI, you’ll see the account page right after you load MetaMask.
Click on the account name (the grey area) to copy your address.
Old User Interface
In the old UI, click on the three dots to the right of your account name to open drop down menu.
Click “Copy Address to clipboard” to copy your address.