Hello, that’s a great question.
No, unfortunately a dehumidifier won’t kill fleas.
Low humidity does kill immature fleas. Eggs and larvae cannot survive below 50% relative humidity. However, these stages live within protected environments where temperatures and humidity are kept moderated.
Most immature fleas within homes live at the base of carpeting. The carpet canopy creates a micro-environment and protects the young fleas from fluctuations in humidity. So, the overall surrounding conditions of the room will have little impact on the micro-environment and the fleas contained within it.