In a lesbian couple, where only one is the biological mother, is the other lesser I'm some way?
In a certain way yes, in other ways no. Both mothers will be the parent, both will have the bond with the child, but only one will be the mother, will have carried and nurtured the child in a way the other won't have and nothing can undo that. I am also not sure if lesser is the right word, multi faceted maybe? That extra dimension you are denying exists but 99,9% of mothers who gave birth will say is there, as will the children in question.
I always wonder why people are so adamant to deny this exists, when there is so much research to show it does. Usually these people have ulterior motives, like being adoptive mother, mother through surrogacy, men.