The added weight may be a lot of emissions over thousands of bikes/batteries, but per bike its probably negligible. The only part that is usually not manfactured locally is the battery, which can be shipped in bulk on cargoships for very minimal CO2 per-item (and more space-efficient than a bike frame).

This is for food, but the point is the same; transport is done in such high quantities that very few food items here even register as significant sources of emissions. I dont think this chart is super accurate but I hope that gets the point across.