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I'm sure it'll be entertaining and worth watching.
I predict their conclusion will be "we can not detect a fuel economy difference between these two cars".
It would take a pretty extreme setup to do aerodynamic testing sensitive enough to detect mud. A wind tunnel would do it, but they wouldn't take a car to a wind tunnel for a simple myth like this. Short of that, you'd need high speed bidirectional coastdown testing on a totally windless day. Even if you accounted for airspeed, small crosswinds would skew your results beyond utility.
As to the effects of mud: It tends to increase the car's frontal area, obviously. Cakes of mud on streamlined surfaces will increase your Cd, while cakes of mud in your eddies will tend to decrease your Cd. A muddy truck with its panel gaps and hood cowl filled with mud will probably get better gas mileage, but unless your aeromodded Metro gets a teardrop-shaped tail made of mud, it's likely to get slightly worse gas mileage when muddy.
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