Yes the zombies created by Syphon Flesh will indeed receive counters from Mazirek.
So first your opponent(s) will sacrifice their creature(s), this will trigger Mazirek, but before those triggers can go on the stack you finish resolving Syphon Flesh, by creating the zombie tokens. Once Syphon Flesh has finished resolving, then the Mazirek abilities will go on the stack, putting +1/+1 counters on all your creatures, including the newly made Zombies.
Syphon Flesh
If I have Mazirek in play and cast Syphon Flesh and my opponents sac 3 creatures, do my zombies from The Syphon Flesh get plus 1 plus 1 counters?
So first your opponent(s) will sacrifice their creature(s), this will trigger Mazirek, but before those triggers can go on the stack you finish resolving Syphon Flesh, by creating the zombie tokens. Once Syphon Flesh has finished resolving, then the Mazirek abilities will go on the stack, putting +1/+1 counters on all your creatures, including the newly made Zombies.
Hope that helps.