Your map is not supposed to be accessed that way, it’s indexed by keys not by positions. A map iterator is bidirectional, just like a list, so the function you are using is no more inefficient than accessing a list by position. If you want random access by position then use a vector or a deque.
Your function could be written with help from std::advance(iter, index) starting from begin():
auto it = myMap.begin();
std::advance(it, index);
return it->first;