Is duct tape okay to use for insulating very small, low current wires?
Low voltage lets you get away with all sorts of things ;^)

Why? - Because they won't burn your house down.

Any sort of (non-conductive) tape is fine - use what you have, without misinterpreting that in silly ways if you happen to have a roll of aluminum foil tape... Duct .vs. gorilla is pretty much "gray or black" in this case - does not matter. Clear packing or office tape would also work. Masking tape would work for a while, then dry up and fall off in the middle of your Carnegie Hall performance.

If your isolation tool arsenal includes a hot glue gun, you can make a neater and more durable fix (I rebuilt a LOT of laptop power cord plugs "better than new" back in the day with hot glue and popsicle stick to replace the "custom molded plastic" at the end of the job. I did solder those, but they were carrying more power than your keyboard sustain pedal wire.)