What does du kanst echt sau qut singen Das is der HAMMER mean?

It means that you can sing very good and that YouTuber or whatever it was thinks you are absolutely great :)

Das ist der Hammer is a colloquial expression. It can be used to express both positive and negative surprise, disbelief or the sense thatsomething is brilliant or wicked. In this case I would translate Das ist der Hammer as that is brilliant (adjective hammermäßig)