I had the exact opposite experience, I didn't even know that the tier meter could go down for the first year or so that I played, up until I started playing a lot more with 'mechs I suck at.
And it doesn't make a lick of difference beyond it being slightly more likely that someone who isn't you will carry. You don't need to be negative, but you need to go in with the assumption that if you don't carry a lance's weight or more your team will lose, and play to that. Obviously if the team is in comms and working together that's an exception, but if it's dead silent nascaring or whatever you gotta do the lift. Also, tier MM is wider than you'd expect: I've been matched repeatedly with people that I know for a fact are sat at the tier cap in tier 1, and I'm still only midway up tier 3.
Damiac, one thing to keep in mind with lights is that, since match score heavily orients around damage and kill assists, you have to play a lot more carefully in a light to get a good damage count but can also stay alive long enough to rack up assists (if your team doesn't instantly fold). 'Course you can also do stuff like running up to a forward vantage point (on certain maps) and hammering R for the scout points, but the easiest is to make sure to get some damage on as many targets as possible (for assist points) and try to stay alive while doing more damage.
It's especially easy with a sniper light, which is why I recommend the Raven-3L so much: super long range, very difficult to pin down and kill. With an ERLL/PPC light you should manage to get at least a 50-60% assist ratio once you know how to handle the 'mech, and it's often quite easy to get a full 12 assists (minus however many kills you secure) on good maps.
And assists are granted for any damage, even if you graze something at 1400m with an ERLL, while giving comparable rewards to kills (though without the extra from destroying components &c. that you'd get for bashing someone's 'mech to bits). Still not sure if it counts assists from enemy deaths that occur after you leave a match, so stay and spectate until the end - you can help your team, too, giving them call-outs and relaying information about what enemy 'mechs are left and where they're damaged.
But yeah, comms are actually really important. If you don't have one, think about at least picking up a cheapo boom mic to wear around your neck or something, even if you're just calling out obvious targets for focus fire or letting people know where that ECM light was heading you're helping keep people organized. PuG play in MWO is different from a lot of other games in that most people are actually willing to work together if someone steps up to call the drop.