You bought no survivability* on a caster whose big damage ult requires him to be right on top of the entire enemy team. You then complain about a lack of slows on your team. Guess what you should have bought instead? Hint: It's Rylai's.
*I'm guessing you shifted your items around some so you could press 1 to use zhonyas and with 10 kills and 30 assists you are kind of bound to be shitting gold so zhonyas might not have been as big a waste as it seems (zhonyas is usually a bad first item because spending the gold on it later means that the % based passive earns you more AP for the gold spent, in the earlier game you can get better returns for the same money), but wardens mail seems pretty weird too, since it kind of woks best of people who can stand around and get hit a couple times, whereas with fiddlesticks you are looking to wait until they are all softened up some, distracted, and have most cc on cooldon to explode in an crowstorm as many peple to dead as fast as possible 9rylai's helps keep them inside crowstorm too. If you were going for randuins which is sort of ok and another way to keep people in crowstorm you should pretty much always buy the heart of gold first.)
If for some reason you HELLA don't want a rylai's scepter a banshees veil is another good al purpose defensive item with some HP/Mana MR and the sweet stun-eating passive. Making sure you have your veil up whe you crowstorm into a teamfight means that they will PROBABLY waste their first cc attempt wiping your shield.
also a lot of people will tell you that if you arent playing fiddles with a mejais at 10+ stacks youre doing it wrong but I can see not wanting to rely on kilstacks with a champ you arent too familiar with
Also personally the situations in which I don't buy mercury treads on every single champion are so few and far between they aren't really worth mentioning.
It's not like the items you bought are trap bad items like malady and warmogs but there are more effective buys for fiddlesticks.
Also If you want CDR spirit visage is a pretty sweet item because it's cheap, has hp mr cdr and it makes your drain heal you for more.