I've got you.
"Is Zilean Mid a decent/somewhat meta pick."
First thing: "Meta" does not mean "good" or "strong" or anything like that. Few people play Zilean and he is rarely played mid so Zilean mid is not meta. Zilean mid also has a pretty good win rate, so he could be decent*. You would be fine to play him there.
(I must note, I know that doesn't mean a lot, especially with picks that are that rare, but it's worth taking into consideration.)
_ "Who would counter Zilean then? Would it be someone with stuns like Morgana? Or a heavy damage dealer like zed?"_
Neither actually; not those reasons. Zed is countered by Zilean and while Morgana does pretty well against Zilean its not for that reason. What counters Zilean is champions who can deal damage more reliably to Zilean that don't all--in and champions who don't need to play against Zilean, to deal with him, to win. Champions that work in that play style deny his strengths in mid by them being avoided simply by them by being not applicable to their play style.
"I was wondering if he is viable to take ranked when i hit thirty or if it would be considered mostly trolling?"
I've been accused of trolling or blamed for playing Annie Mid. So yeah...
Don't listen to those fools who accuse everything of being trolling.
P.S. Also, if you are a new player, consider not playing Ranked when you just hit 30. You probably won't enjoy the results.