Not quite. The ratings are about risk/reward, but they are also about difficulty. A higher-rated contract is more difficult, hence riskier, and with more reward than a lower-rated contract. I fully expect to have to go to lower-rated contracts from time to time when my best 'Mechs are undergoing repairs.
Let's take a completely made-up example: A one-rated contract might see you facing off against a lance of light 'Mechs; easy if you're dropping with mediums. A two-rated contract might have you fight a lance of mediums and a couple of vehicles; that's more interesting for your medium lance. A three-rated contract might have you drop against two lances of heavies and mediums, with vehicle support - now that's a challenge for a lance of four mediums. Four-rated? Toss in an assault or two. Five-rated? Assaults, heavies, vehicles, turrets, the works. And with one or two waves of reinforcements just to spice things up.
One should probably also keep in mind that there will be a selection of mission
types; plain-old battles, base defence, base assault, escort missions, rescue missions, and so on.
The good thing about having a selection of risk/reward/difficulty missions is that I can choose what I want to play at any given time. Do I feel like taking on a harder mission for larger rewards today or do I want to take it easy and earn a little less? Do I want to play a base assault or do a rescue mission?
Now mix in that my 'Mechs might be in a range of states of repair and that my MechWarriors may be recuperating, and it becomes an interesting decision: What can my current company strength handle? Just because I could take a three-rated contract with my best 'Mechs/MechWarriors doesn't mean I can do it with my second-stringers.
I for one do not think I will be bored at all with this game