The musical “Kimberly Akimbo” wastes no time in staking a claim to both exuberance and the weight of the world — as well as about a thousand other feelings that its titular lead, her teenage friends and, arguably, the rest of us are awash in. That David Lindsay-Abaire and Jeanine Tesori’s musical about a teenager with an extremely rare disease and her dysfunctional parents (less rare but oh-so challenging), can be soul-affirming without losing its edge and ouchy-ness is an achievement, one that speaks to the elasticity of the musical genre.