I have had lots of matches with a load of damage where we lost, because the other team gathered more resources or our base got capped.
And of course there were the usual 'LAME!!!' comments in the chat if the base got capped. But someone on the other team (maybe a medium) made his job (read the objectives, capping is a legitimate means of winning) and our team just forgot about our job (capping or defending the base). And more often than not the guys in their assaults who complain about your 'medium Mech ruining the chance to win' play like a {Dezgra} and try to run down that Jenner who nicked their torso armor.
Damage number sometimes say very little about how well you played. You can take a Muromets, put in 3 AC5 and 4 med lasers and put up damage numbers of 1K and more regularly, but maybe your aim is not really that good, so you spread it around on the enemy Mechs and fail to kill them quickly. You can take an Atlas down by dealing 400+ damage (if you spread it round enough) over some minutes, but ideally you hit the head once or twice and he is down in seconds. First method looks better on paper, second method is better for your chances of winning.
Usually I try to take them down by taking out the CT or sides, depends on XL engine or not. (I said try, not manage...)
I have more respect for someone making 5 kills with 200 damage than for someone with 1 kill and 1000 damage.
Although: one of my worst games was with 1 kill and 20 damage. Hit a locust in the back with my AC20 (in a medium, btw.) but failed to notice the Assault in my rear

If you take a long bringing down a mech he will deal out damage at the same time. If you do it quick, he will deal less damage.
How fast you can take someone down is not a matter of your Mech alone, it´s a matter of how you play.
If you play badly in general, it will not matter in which Mech you arrive. If you are playing good you are good in (almost) any Mech.
And first of all, this is a game, do what is the most fun for you (without ruining it for your teammates).