Hello there, I used to be a Karthus main in season 2, and I was a Gold elo player. My other lanes were very average save for support but it was my constant success with Karthus that brought me to Gold and allowed me to stay there by playing this champ.
Once again, I'm not a high elo player so take my suggestion with a grain of salt.
In my opinion, Karthus is not seen a lot anymore, and its odd to say that when he was a great and frequent pick and ban in high elo and tournaments.
I believe the main reason for this is because the current meta of the "league of warmogs" With everyone going so tanky, the high usage of escape champions, and the squishy nature of Karthus, it is hard for him to deal the damage even after he dies.