No, this is balanced.
Compare this to the deathstalker.
They both are 0 ATK and needs buff, making duo.
And on avg, Deathstalker and Dune Scropion adds 2 poison counter each turn.
Yes, if deathstalker is dead there's no more poison unlike dune scorpion.
That's true, but +1 cost for dune scorpion
Furthermore, neurotoxin effect is not stacked.
Even you played two dune scorpions, opponents get 1 counter when playing cards, not 2.