Part of the problem here is that while you're right in terms of creating lift, there's a fundamental problem you're missing.
Battlemechs aren't aerodynamic. Like, at all.
Even the most curved of them, like the Wraith, is a 55-ton wad of vertical metal. Limbs and such all over the place, and as I said, standing vertical- which means during a jump the main form of the unit is creating a huge amount of drag. (The same issue would be true to a lesser extent in LAMs, in which the main body at least is now flush with the travel direction and thus not creating as much drag, but still suffers the limb problem)
So while a form like the Flying Pancake would theoretically be more efficient for lift, from a flight standpoint it's six of one, half dozen of the other- neither that shape nor that of a Shrike's wing would create enough lift to truly make a major difference in the flight characteristics of a Battlemech. If it's not going to really matter, might as well make it look good, right? (And far be it from me to tell the Jade Falcons that their designs look silly, after all- their reactions to being called out for silliness tend to be a little... pissy.)
At the end of the day, yeah, you're absolutely right about the shapes when it comes to aerodynamics of an actual aircraft- in the case of the Flying Pancake, you have a lifting-body, in which the body forms the lift surface, which is always the most efficient form of lift (the B-2 is a very different-shaped but apt example as well). Here though, it's the equivalent of asking if a spoiler will help the performance of a Toyota Camry- yeah, I mean it can't hurt the cause, but it's not really going to help either. ;)