What counts towards the goal in Mallcrashers?

You can destroy lots of stuff:

• Mall's black and white signs
• Stores glasses
• Glass runways/railings
• Glass roof
• Glass shelves in some of the stores
• Wine bottles in the wine store
• Vases and poteries at the antique store
• Sport car's windows in the showroom
• Shoe box stacks (tip: directly shoot the top of the stack)
• Computers and headphones at the Pear store
• ATMs if you have a saw
• Clothing stores and gym can be burned down with a gas can (cf. mission's assets)

Note that trip mines can prove useful in this level: simply plant one in the middle of an area where you want to blast stuff (liquor bottles for instance) and shoot at it.

You can add gas cans to your assets in this mission which can be used to set the gym, clothing stores and a few other places I think on fire.

Another good tactic, and one which will earn you the Steam achievement "Shoot the Glass!", is shooting the upper level windows upstairs. They are just below the skylight, and are panel glazed.

I also tend to shoot out all the black and white shop signs and mall brand signs.