Birkenmocks are any type or brand of fake Birkenstocks. Hence the stem word "mock". Birkenmocks are cheaper than the actual hippie shoes and can often be found on the feet of cheap/broke activists, treehuggers, peacniks, along with many lazy high schoolers. These shoes say "I'm a hippie on a budget".
Mari was too broke for the real thing, so she bought some Birkenmocks from Payless.