I honestly don't think karthus is bad at all.

http://champion.gg/champion/Karthus/Top

He currently has a 57% winrate when played top and almost a 70% winrate when you have over 125 games on him.

It seems that when you know what you are doing, you can be quite successful with him...

Unless there is a soraka on the enemy team... then you're boned.