Again, I'm not gonna argue the merits of Meganium vs Serperior (or any other Starter) in this thread. PM me to talk about that.

I never said Serperior is a bad Pokemon (though it is NU right now, along with Meganium, Torterra and it's BW Starter bretheren,) it just can't catch up with some of the more potent threats of the metagame, particularly in OU.

In the NU tier, where it resides, it has to worry about Swellow, Magmortar and Shell Smash Gorebyss (who will outspeed after a boost and often carry Ice Beam.) NU isn't very kind to Grass types not named Exeggutor right now. The best Serperior can do is to work with Gorebyss to set up screens.

RU is a tier where the likes of Charizard, Typhlosion and Entei run rampant. Not very welcoming for Serperior right now. Furthermore, with Sceptile (and its better offensive movepool) available in the tier, Serperior is kind of outclassed.

UU has Heracross, Darmanitan and Chandelure in the top 20, not to mention Serperior would have to compete with other Grass types like Roserade (the current #1) for a slot. That's a definite upward hill to climb.

OU is just not an option for Serperior right now. If you can't compliment/counter Volt-Turn or benefit from Weather, you have no place in OU at the moment. Top threats like Ferrothorn, Scizor and Dragonite can completely wall it. When it's not dealing with the top threats, it has to take heat from the three weathers of OU, all of which are detrimental to it (even sun, the quintessential Grass weather, just guarantees it gets KO'd by a Fire move and nothing else.) Then there's the issue that what it currently brings to a team (Subseeding, Screen support) is either outpeformed by a better Pokemon or doesn't provide enough benefits to be considered on most teams in OU.

Serperior has no business in Uber.

It's not that Serperior is a bad Pokemon, when analyzed individually, it's obviously quite solid and when Contrary gets released, it'll be a major powerhouse, but the metagame is just not being kind to it right now. Everything Serperior despises is what's popular and as such, it just can't keep up.

A better stat spread and a wider variety of moves (if only support moves) would go a long way to remedying it's shortcomings (of course, this could be said for pretty much any Pokemon but I digress,) but alas, that crappy movepool and decent but not gamebreaking stats is what we have to deal with for now.