Why does acetone have a greater dipole moment than dimethyl ether?
The ether is fairly symmetrical though there is a bond angle of ~111°, "exposing" the oxygen on the side (and its electron cloud).  Acetone, however, has an oxygen off to the side, with its electron cloud offset ~120 picometers (pm), making it more polar.

Remember that oxygen is a strong electron acceptor, so that there will be a "lump" of electron probability surrounding it.



[Sorry about the hodgepodge illustration!]