[aerohead]

Since the sunshade 'resides' within the 'forebody' of the truck, and the ground rules for road vehicle aerodynamics states that fully-attached flow exists without the 'need' of any added aftermarket device, then what enters the mind is; if you arbitrarily introduce some 'structure' into a region of perfectly-adequate airflow, are you running the risk of 'degrading' that airflow.
So far, the available literature says 'yes.'
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If the 'shade' were on the other 'end' of the cab, it might be 'angled', to create a primitive 'cab-wing', which has been verified by empirical tests at Texas Tech University to reduce drag.
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The only 'caveat' to this 'sun shade' question hinges on:
A) all driving is done at 'low' velocity
B) the sun is near zenith
C) it's Summer, and your running the AC
D) the 'shade' lowers the 'solar gain' into the cabin
E) which reduces the time the compressor is running
F) which cuts the engine load by around 3-horsepower
G) saving fuel!
As long as you don't get on the highway, where aerodynamic forces are greater, the sunshade could be of benefit.