Incandescent light bulbs do become dimmer as they get older, but that is
NOT due to dust accumulation on the light bulbs. It's entirely due to
the tungsten metal atoms from the filament coating the inside of the
bulb. So, this is something that cannot be removed by washing the
outside of the bulb.
Here, prove it to yourself: When you replace light bulbs that have
spent their lives in one position; such as vertical in the case of
freestanding floor or desk lamps, or horizontal in the case of bathroom
and bedroom light fixtures, or upside down as in the case of porch and
hallway lights, inspect the bulb when you remove it from the fixture.
You will notice that the top of the bulb always has the most darkening
on the inside of the glass. That's because incandescent light bulbs are
filled with an inert gas (most often argon), and convection of hot argon
gas off the hot tungsten filiment carries the tungsten atoms upwards,
thereby causing the topmost part of the bulb to darken the most.
Washing your light bulbs won't eliminate that darkening that happens
inside the bulb.