Can non-creature permanents have +1/+1 counters?
Lands (any permanent, really) can have +1/+1 counters just fine. In fact, Llanowar Reborn enters the battlefield with one! The counter(s) will not go away just because your blinkmoth stops being a creature.

Of course, the +1/+1 counter doesn't do anything until the land becomes a creature again, but the +1/+1 counter will still be there.