Grim Poppet dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it. This will be removed by its static ability, which will result in
Grim Poppet having two -1/-1 counters instead.
Resolve the
Bloodflow Connoisseur ability.
Repeat.
Notes
You can remove -1/-1 counters from
Grim Poppet and put them on other creatures, if desired.