'Skinny' is a loosely applied term and none to helpful really.
A good idea of what a healthy well balanced animal of your species looks like and a knowledge of what to look for (early indications of emaciation/ill health) will serve you much better.
Size and proportion are relative, chams being a laterally thin creature, means some find it difficult to judge.
Ribs visible during stretching/flexing can be taken for 'skinny', particularly in photos.
A normally relaxed animal shouldnt have ribs showing. Limbs should be relatively well developed in proportion to body size, neither too thin and spindly, but not odd thick either.
Tail should be nicely tapered from the spine with a plump appearance at the base,
casque (if any) should be nicely filled out, but neither concave nor particularly convex.
You should not see 'outlining' of the spine (concave contour) but sides should taper to the spine without an inverted ridge running along underneath.
Head should have no particularly odd looking concavities (around the jaw hinge or eyes).
Its difficult to describe a perfectly healthy animal and hard to point these things out without photos for comparison.
Bare in mind, emaciation/thiness is often seen along with symptoms of dehydration, which draw our attention and magnify the emaciated appearance, for example sunken eyes, permanantly wrinkled skin.
I have not yet come across a better formular for monitoring general health than measurements of weight on a monthly basis, more often for hatchlings/juveniles, and ofcourse the golden rule....Observation daily.
P.S there appear to no photos/video posted or attached to this thread at the time of writing. ???
It also depends on what type of chameleon species you are talking about. For example, veiled chameleons have more of an arched back and therefore appear to weigh noticeably more. Whereas a chameleon such as the outstalets or verrucosus are visibly "skinny" and have flatter backs. I was concerned my verru was too skinny for his age. His weight to length ratio appears to be much less than my carpet and rudis. However, my vet assured me that the chameleon was in fact "lean" and not underweight. So like I said, you first must know the typical appearance of your species and then you can go from there. Please post pictures if you have any.