Okay, so, did you actually get a punishment or warning for this, or did you just see people say "reporting you after game"?
If it's people claiming to report, don't worry about it. It's actually been looked into by Riot, and most of the time people who announce their reports either A. actually don't do it and are just being shitheads trying to intimidate you into doing what they want (and are probably worthy of actual reports), or B. report on such a trigger finger that their reports are actually ignored by the system.
Most importantly, it takes consistent reporting from credible reporters to actually recieve any kind of punishment or even warning. I've never, ever seen anyone actually get punished solely for playing off-meta picks. Key word solely - I've seen a number of players in my years here show up arguing that their weird picks are what got them banned, but then you see them going 0/20 with six boots in games in their match history, or their chat logs are revealed by riot, and it's not pretty.
So if you've actually recieved a warning/punishment after playing a game as Morde, this isn't because of that game (one game shouldn't ever net you a punishment) - it's because you've built up a consistent pattern of negative behaviour prior to this point.
TL;DR - You don't have to worry about reports for what you pick, you won't face punishment. Unless you're also being a jerk in which case you will regardless of who you pick.