It's not that the Café doesn't have metal cutlery. Here, Bray brings Trudy a bowl of food complete with shiny metal spoon. And when Trudy names her baby, and Salene makes a vegetable pie, Ryan cuts the pie with a metal knife, and everyone at table has a metal knife and fork. See Ep.12.

But for some reason, the Mallrats begin to prefer plastic cutlery. You know, the sort you get on airplanes and breaks very easily. So that's why Trudy looks real stupid when threatening Bray with a white plastic knife.
And it looks even more stupid when Bray has thoughts of stabbing Lex - with a plastic knife - and with the blade towards his own face