Would you look at that!

Just a little background information, Riot tries to keep all champion win-rates just above 50%. Not many go above 52%, and even fewer go above that. When a champion starts to hover around anywhere 53% - 60% win-rate, or perhaps even higher, is the time that Riot starts looking for nerfs or places to trim power from. The same is true of a champion sub 50%, they look for places to add power. I'll list a few examples below:

http://champion.gg/champion/Shyvana

Now here we see Shyvana has quite a low winrate, and perhaps she could use some love. As we can see here:

http://na.leagueoflegends.com/en/news/champions-skins/champion-update/ability-update-shyvana

Riot agrees. Next patch they are giving her a bit of an ability update to push her in a better direction and make her more balance-able.

Now on the reverse side Kog'Maw had been a problem through out the season, with win-rates soaring about 56%. Because of this they semi-reverted his marksman rework.

http://na.leagueoflegends.com/en/news/game-updates/patch/patch-621-notes

Here is some context for that as well:

http://boards.na.leagueoflegends.com/en/c/developer-corner/NZK9A5NP-upcoming-kogmaw-changes

As we can see, just like Shyvana, they wanted to push Kog'Maw back into a healthier spot. When we look at his win-rate in the current patch, which is significantly lower, we can see that is was successful in keeping him more in line with other champions and is still improving as players continue to get used to the partial-revert. Perhaps Riot will even give him a tiny buff or two to help out his ADC without pushing his midlane out of line.

http://champion.gg/champion/KogMaw

So with these above examples in mind, when we take a look at Karthus again, we can see he is sitting at a perfectly normal win-rate. Another thing to consider is over 50% of players who play Karthus only have 1-15 games on him, and when we look at that group specifically his winrate is actually a shockingly low 47%, a full 5% below his average win-rate. One more thing is that his average pick-rate hovers around 1% while a much more common midlaner, Lux, is picked nearly 10% of the time!

http://champion.gg/champion/Lux

With all of this information in mind, we can see that Karthus is in a perfectly healthy, normal spot. No need for a gut here, and hopefully you have found all of this information to be of use! :)