Yes, it would be overpowered to change any fire spell to any other damage type. Fire spells and effects deal more damage because there are more creatures which resist fire. This is an important aspect of how they are balanced. You can also see this with magic weapons, where the flametongue deals +2d6 fire damage and the frostbrand only deals +1d6 cold damage. If you wanted to create an Acidbolt or Acidball, they would need to deal less damage in order to be balanced.
It is true that there are more creatures that are resistant/immune to fire than there are to cold, lightning, or acid. However, there are also far more creatures that are vulnerable to fire than to any other type of damage.
Also, fire spells don't always deal more damage than other spells. Lightning bolt deals the same amount of damage as fireball. Spells like chill touch, ray of frost and shocking grasp do a bit less damage than fire bolt, but they also have secondary effects to compensate (negating healing, slowing speed, denying reactions, etc). I don't think it has anything to do with their damage type being something other than fire.