At this point, I don't know why those truss need to be there. On one hand, they do have fantastical materials in TOS, on the other hand, the interior rooms like the cabin, briefing room, and engine room all contain some type of visible framework.
Perhaps the truss functions as part of the framework to keep the corridors "stiff" in the same way the other rooms have their frames and braces? Or they are physical bridges carrying circuits from one section to the other since it seems like on the TOS Enterprise all the pipes and circuits run inside a wall or under the truss (the pipes) and the floor itself seems rather thin (or has nothing running below it.)
I agree with this completely. I assume the actual deck between the floor of one deck and the ceiling of the deck below to be just a few inches thick and the artificial gravity is produced by plating contained therein. Piping and wiring conduits would be located in walls (which tend to be super thick...in fact to contain the stagehands who operate the doors, but in-universe, I assume to accommodate utilities that the floors are too thin for.)
The trusses are IMHO the skeletal structure of the ship. Piping and wiring is run mainly around the corridors so the trusses appear somewhat bulkier over the corridor as they cover piping which runs into the walls but not along the trusses in the individual cabins and compartments.
Why the funny wavy Mylar panels? I say it's decoration pure and simple. Blssdwlf
makes a good point when he says that if these are the gravity generators, why does every other area lack this kind of device? But, I would also point out two other data points. The shuttlecraft is a compact enough vehicle, that it must use some sort of gravity plating as I have described above. (And is described in numerous non-canon works of technical fiction.) There's no reason a similar system wouldn't be in use aboard the mothership. Also, we see the same wavy Mylar patterned panels in other places, most notably the transporter chamber. Some have suggested that this is some sort of future-y material/equipment vital to the transporter's use, but we do see transporters which lack it, off the top of my head, the 2-man unit in Mr. Lurry's office in "The Trouble With Tribbles." Also, what mechanical/electronic purpose would the corridor trusses and the walls of the transporter chamber have in common? In real life, it was an exotic looking shower curtain cut to shape and applied to the surfaces to dress them up, and I propose that in-universe, it's basically the same thing.
Also, since we're referencing "In A Mirror Darkly" I would point out that when evil Archer orders the gravity turned up to 20g to crush the "Gorn" we see a plate reading "GRAV GRADIENT PLANE." I posit that since this plate is on the floor, it possibly relates to equipment found within the deck itself.