No they arent. The convention ont he fedi is to keep people at the bottom and the first person tagged at the top indicates who you reply to.
Some servers (perhaps yours) will auto tag people, others dont do that, it depends on the software your on.
Even when it is automatically done the convention is often to leave it anyway as it gives an explicit indication as to who is directly being responded to, something you can do if you leave off the OP as the OP wont know when you are addressing them by moving their name back tot he top. So its a convention usually maintained regardless (though you dont have to).