I never seen a Karthus toplane do well. They either feed or get hard carried. His Q is ridiculously easy to dodge, so is the wall, e doesn't bring enough dps to a sustained fight, and his ult can be negated by a bunch of items. Even a support like Soraka would be a better laner than him.
PopcornBunni2 votesTry using Q to deny cs or get a trade and you'll find much more success with him [slayer-pantheon-thumbs]
Ben Sharpio2 votesand then something crazy happens. an immobile champion gets ganked out of the game
xXRaichopXx2 votesAnd then something crazier happens...
Karthus buys a 
!!!!!!!!!!
z63XnLCMwX1 votesTry using Q to deny cs or get a trade and you'll find much more success with him [slayer-pantheon-thumbs]
His Q doesn't take much effort to dodge and even if you take the hit, just take it with minions so the damage is reduced. Toplane champs like 
would dominate the lane. MId lane, I think Karthus may work, but not top lane against meta top lane champs.
oSEXYPLATYPUSo1 voteshow is it he ok mid but top is different?
he still has his R
and its all about match ups and team comps
Warlord Rhinark1 votesWhy would you take him top lane in the first place?