It's easy to precisely define what lukewarm is, especially in cooking, because it implies that fluid should be body temperature. I.e. that you cannot feel any temperature difference when putting your finger in it.
Lukecool on the other hand would be hard to define precisely.
I would think that a finger, being an extremity, would be cooler than core body temperature - and not by a precise amount, either. "Lukewarm" by this definition would be cooler on a cold day, and vary depending on whether your hand was in your pocket prior to measurement.
If you had your hand in the pocket or have been outside in freezing temperatures you usually cannot determine any lukewarm temperature in a precise way. It wouldn't make sense to use the term lukewarm or lukecool in those cases IMHO.
Lukecool on the other hand would be hard to define precisely.