Deliberate drunkenness is a sin, but in order for it to be mortal, you have to understand that it’s a sin and do it anyway. If you just kinda loose track of how many you’ve had and find yourself drunk without the premeditated intent to become drunk, it’s not a sin. If you’re deliberately trying to inebriate yourself though, then it’s a sin against the 5th commandment, because you’re deliberately causing harm to your body.