The truth is that there are rarely any bad mechs in the MWO game. For the average player, or PUG (pick-up group) player, almost all mechs are "good" or "viable". Each mech has their strengths and weaknesses. A skilled player can make a difference to a team in any mech - this difference or ability to help a team or ability to win a game is called "carrying".
However, at the competition level of play, there are viable mechs and non-viable mechs. Competition level denotes the highest level of min-max'ing where players try compete with the "best" mechs and their "best" configurations to do the most amount of damage in a limited period of time. However, what defines "best" is strategy and is a concept called "META" - META exists in any team game.
http://en.wikipedia....wiki/Metagaming
In MWO, as previously mentioned, the meta is the ideal min-max of units with certain weapon layouts that effectively counters the most popular / common units and layouts AT THE MOMENT.
The key to the meta is the "AT THE MOMENT" condition - that is, the META constantly evolves and rotates as new units and weapons are added to the game. Eventually, you may even see a return to an "older" META that counters the "newest" META.
Hence, it is important to realize that every mech may be useful or "good" at one time or another. There are rarely any bad mechs in PUG play. In competition play, for the current META, there are viable mechs and viable layouts and non-viable mechs and non-viable layouts.
This every-changing META is why serious players have and have used almost every mech and every weapon in the MWO game: we have numerous mechs and for each mech, numerous layouts. ( By analogy, this is why Pro Players in League of Legends have and have played every champion! )
MWO is a strategic first-person shooter. Strategy and understanding the META is key to mastering the game!

I hope this helps explains why there are very few "bad" mechs in MWO game.
MavRCK
-be a better pilot.
PS:
For those who desire. Here is my list of what I consider the "best" in each weight class as of 12/12/15 for competition-level play:
Light - Raven 3L
Medium - Hunchback 4SP
Heavy - Catapult K2
Assault - Atlas DDC
On second thoughts, I will start a list of "bad" mechs for competitive play using a 2-2-2-2 (2 light - 2 medium - 2 heavy - 2 assault) meta:
Commando - all variants - 2D - gives up 10 tons which makes it too slow when fitting in ECM and 3 streaks +/- 1 SL / tag.
Jenner - K - not bad per se, but why choose over the D or F?
Raven - 2X, 4X - too slow, no ECM
Cicada - all but the 3M - no ECM
Centurion - as per the K - not "bad" per se, but why choose over a hunchback 4SP?
Hunchback - the 4SP shines as the best of the bunch.. why choose other variants?
Dragon - like the centurion and the Jenner K, why choose over a Catapult or Cataphract
Cataphract - 4X, really does nothing that the Catapult K2 does better.
Awesome - all variants - so not awesome - may improve when PPCs are buffed ( a big maybe)
Atlas - K and D are tied - the DDC is the best due to ECM, 2 ballistic slots and 3 module slots - the RS is the next best with 4 laser hard points in the arms - why choose the K or D when there are 2 laser hardpoints into the torso? However, the K does benefit from double AMS.
Will update!

GL HF HH! (Good luck, Have Fun, Happy Hunting!)
Edited by MavRCK, 16 December 2012 - 02:02 PM.