No, SKlauncher itself is not a virus. However, because it is a .exe file and is often used by players who don't own the game officially, some antivirus software may flag it as "unwanted software" or "crack tool." If you download it from the official skmedix.pl domain, it is safe to use.
The only safe place to download SKlauncher is from their official domain. Avoid "mirror" sites or YouTube links that lead to file-hosting services like MediaFire or Mega. sklauncher download