Is this an acceptable way to fix garage roof condensation?
That material looks very similar to what I used to cure a condensation problem with my air ducts.  I live in a humid area and the attic is not insulated from the outside. So using the a/c would cause condensation to form on the ductwork and drip on the drywall below.  Your situation sounds similar.  If you line the metal roof with that material, it should reduce the amount of moist, inside air coming into contact with the cold roofing (where it sounds like it is condensing).  

I'm not sure how easy it'll be to apply a flexible material like that to the underside of a corrugated roof, however.  You might want to look for a hard foam instead.  As for the mold concern, if you seal all the edges of the insulation with tape (use the silver metallic kind used for HVAC), that should prevent air exchange.  The temperature between the underside of the roof and the roof-side of the insulation will be less extreme than the inside of the garage or the exterior of the roof and less likely to produce condensation.  That said, it might still be a good idea to inspect it occassional.  And all of this is assuming that you really do have a condensation problem and not just a leaky roof.  - Good luck.