I get it (or some kind of sweep off the reaction) all the time. The only thing really bad about the kimura from bottom half is the setup forces you to be in a position that doesnt have many options.
Anyway, the kimura from bottom half is about winning two races.
The first race is to get both your shoulders anchored to the floor before your opponent can pull you up.
The second race is to get your ear next to your opponents hip to get the leverage to break your opponents grip. Once you win this race you will get the tap or the sweep. Even if the opponent gets sidecontrol as long as you got what you need you can tap him from under side.
it is not low percentage, just neglected. Besides, people with good halfguard fundamentals never allow themselves to be in a position where the bottom half kimura would work best.