Is a biplane without dihedral more stable than a low wing monoplane without dihedral?
Narrow answer: Yes, it is.

But no aircraft designer in his right mind would build an unswept low wing aircraft without dihedral. So this question is misleading.

Please look here why low wing aircraft need more dihedral.

Please read here why a low wing aircraft is just as stable as a high wing aircraft. The high wing does not entail more stability!