When Grim Poppet dies, its undying ability triggers, returning it from your graveyard to the battlefield without a +1/+1 counter on it due to its replacement effect.
Resolve the Shilgengar ability
Repeat.
This clever loop is orchestrated by Shilgengar, Sire of Famine, whose ability lets you sacrifice Grim Poppet to fuel its ritual sacrifice engine. Because Mikaeus, the Unhallowed grants undying to your non-Human creatures, Grim Poppet continuously returns to the battlefield whenever it dies. The interaction is self-sustaining because Grim Poppet naturally enters with -1/-1 counters that cancel out the +1/+1 counter from undying, keeping its counter count clean and its undying trigger perpetually ready for the next iteration.