Does a heavier mouse mean better build quality?

No. Weight historically came from thicker shells and steel plates, but many mice have had weights added deliberately to feel substantial. Shell stiffness — how much it creaks and flexes when you squeeze it — tells you far more about how it was built.

Why it happens

Modern lightweight mice remove shell material rather than change what it is made of. A 50 g mouse and a 100 g mouse are frequently the same ABS; one simply has thinner walls, fewer internal ribs, and sometimes a perforated shell. The heavier one is not better made — often it is just less carefully engineered.

The parts that determine how a mouse actually feels are small and cheap: the switches, the scroll encoder's detent spring, and the PTFE feet. Virgin PTFE feet in particular have an outsized effect on glide, and they cost very little.