It will give you a visual demonstration of the current distribution
along the antenna, as
G8JNJ shows here (scroll down to
almost the bottom of the page).
Conventional incandescent lamps will, of course dissipate your RF
as light and heat, reducing the radiated power significantly.
LED lamps could generate harmonics, and will certainly be
non-linear.
However, many strings of lamps have 3 wires, two of which
run through to the opposite end with no lamps, to allow
multiple strings to be connected in series. Those will be carrying
120 VAC, but will make a better antenna (or feedline) than the
wire with the bulbs. There will still be significant coupling between
the wires, however, so even with the lamp wire disconnected
you may still see them glow
But, while you are up there hanging the Christmas lamps, why
not hang a separate antenna wire under the eaves, where it is
out of sight and out of the weather, and use that for your antenna.
Then you can use it the rest of the year as well. I've done fine
just stapling an insulated wire to the back of the fascia boards,
but you can use a small piece of plastic for an insulator, or put
in an electric fence standoff insulator, especially if you plan to
run more power.