Of course weight makes a difference.
More work is done when mass increases. The most basic of physics.
Imagine a motor pushing a 60KG person vs pushing a 120KG person.
If you don't believe it, fill up six10-litre buckets of water (or find 60KG of weight, such as another person) and walk up a local hill.
Cargo bikes are an entirely different thing to eMTBs but physics still applies. Braking is longer, climbing is slower etc.
Given the cost of carbon just to save a kilo, the $$$ is not worth it. You'd be better off carrying 1KG less of water/food/gear or just losing 1KG weight (free and for a 120KG person, just a loss of less than 1%).
As for motors, everyone has their own story. Look at the weight limits made by the bike manufacturer.
Specialized Turbo Levo Expert had a 109KG limit. This is not just about the motor, but hubs etc. There are threads here like
Am I too heavy for an ebike? - EMTB Forums