Yes you can put a resistor in parallel to dim the LED but there is seldom a good reason to do so since it just wastes power that you wouldn't otherwise waste using just a single resistor in place of the existing resistor, either to limit current to the LED more if existing resistor is in series, or in a current source would be in the driver feedback to limit the output current to the LED more. Usually if you don't want to get that far into it, you'd just put a lower light transmission panel in front (piece of tape if nothing else) or a dome over it or a black sharpie marker to darken it or *whatever*.
If this is the typical complaint I've seen on the internet over the years where you have some widget in a bedroom and a person is convinced (probably wrongly so, that they just notice the LED because they're awake) the reason they can't sleep is a too bright LED from some indicator panel LED putting out a half dozen lumens or less, those usually do use only a series resistor to limit current and that resistor can be swapped out for a higher value resistor to save more power instead of wasting more.
Granted at the drive current for only a few lumens you aren't wasting much if you just throw in a parallel resistor instead, but does it really save any time or hassle? Doubtful, you still have to measure voltage, calculate resistor, already have or order that resistor... same difference as replacing the original with the right value for your needs except for desoldering the original resistor which only takes a few more seconds since you had the soldering iron out anyway to put the new resistor in.