How to specify different margin for different screen sizes using breakpoint in Vuetify

$vuetify.breakpoint.smAndDown

Notice $vuetify

In your case:

<v-flex 
    v-for="card in filteredCards"
    :key="card.id"
    :class="{'ma-0': $vuetify.breakpoint.smAndDown, 'ma-5': $vuetify.breakpoint.mdAndUp}"
    xs12 sm6 md4  
>

Check docs (Breakpoint object)