This is my first post, and I'm just a casual player, so please don't bite.

I was playing on MWS with a UWG Noble Fish deck (Selkie, Meddling Mage, Heirarch, Qasali, Tarmogoyf, Null Rod, counterspells, wastes). My opponent played something like a mox and a Sensei's Divining Top early. I figured I was most likely up against Tez/tendrils/oath.
Anyhow, it was my turn, and I decided to play Meddling Mage naming Sensei's Divining Top. I must have not had other good options, but I wonder if it's better to just wait out a turn or two to gain more information about my opponent's deck before casting the Mage. Maybe Mage doesn't deserve a maindeck spot precisely because of this sort of quandary?
Sorry I don't remember too many more details, besides the fact that I was eventually done in by a Yawg Will into Tendrils.
Yeah, they key with mage is to anticiapte what kind of threats/answers you will be dealing with from the opponents deck, and use it to tactically disarm them. the Top, once played, is no longer anticipated - it's a post-issue.
Mage requires a good grasp of the metagame to use effectively. For example, in your situation I think your read on what the opponent was on was generally a good one. Top doesn;t shw up in much else. So knowing that, what do you name?
Top is not the correct choice - the only time i woudl ever name Top with MM is in Legacy vs CounterTop, where it is a lock piece rather than just a deck smoother, and even then 8-9/10 times I'd name Counterbalance instead.
Do you have other answers in your hand, specific answers to threats (like Stifle or Mondbreak trap vs Tendrils, or Drain/Krosan Grip vs Oath/Tezz)? Is fo, MM naming FoW woudl be a strong play in this situation, as it make it far more likely that you will be able to answer their critical threat. Is your hand full of dudes but no other answers? If so, getting a read on what they are playing is key to using the mage effective as you will want to use it to keep them off the critical threat. Did they lay an Orchard? If so, MM naming Oath is a relatively safe play (yes, it could just be a sign of a transformational board, but mosty of the time Orchard=Oath). Are they being meticulous about keeping UU open? MM naming Drain might be a good idea, especially if you have a Null Rod or such back - either they drain the MM and you've drawn the counter, or they don't drain it and you;ve shut off the counter.
MM is a good maindeck card, but only if you have sufficient knowledge of the metagame and how other decks play out that you can correctly pick what to name.