His ult is a joke these days, every champion can either tank it, avoid it, or heal the damage up. And unless you're fed as hell the damage will be laughable anyways. His whole kit needs to be reworked. Ult needs to be unavoidable/50% true damage
Man his early game is depressing. He is so mana hungry he needs to get mana items but he falls short on AP really hard.
There are two ways to use a Karthus Ult.
To initiate from a distance. Like tell Bot Lane "Go now! I'll Ult." So they are diving in at the same time you Ult. This is where it can be Healed, Negated (Zhonya's, for example) Tanked and all the other stuff you said, BUT you have your team wailing on them also, giving them a nice advantage in that Fight.
Clean Up Duty. You know, AFTER Abilities are blown, Mana is Low, Summoner's are spent and most general ways to escape the Wrath of R are no longer available.
If you use the Karthus Ult in Situation 1, you shouldn't expect Kills... but that you may well have turned a fight for your Team (a good thing). If you use the Karthus Ult in Situation 2, you are damn near guaranteed to wipe up Kills (a good thing).
Also, name 1 Champion in the game with an Unavoidable Ult. Just one, that Banshee's, QSS, Cleanse or a Blink can't solve, that actually Targets an Enemy. You don't think an Unavoidable Ult is... you know... a bit much?
Karthus is doing pretty fine. I don't play Karthus. At all. But this game I was mid, and didn't want to pick one of my usual mid laners, so I took him just to have some fun. This was the result: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2184890514/205363278?tab=overview, plus I got a highlight from the match: http://plays.tv/s/KnNKqp3DGfpg . He currently has the 4th highest winrate mid (54%) in platinum+ ranked. (http://champion.gg/statistics/#?sortBy=general.winPercent&order=descend&roleSort=Middle )
So I'd say he's better then mediocre.
Highest damage dealt on average of any mid laner (and any champion period) 4th highest winrate of any mid lane champion I wouldn't call that mediocre.
