See this question on called shots.
The player is trying to disable an enemy by bypassing their hitpoints. Mage hand is not invisible, can't attack, and is pretty weak. If you are modelling the mage hand as being able to move towards the monster and touch it, it probably notices the floating spectral hand and backhands it away or dodges it with no notable effort as the hand can't fight.
In addition, hitpoints serve as a protection against crippling or stunning attacks that work on the basis of biologically destroying body parts. You can't bypass them by targeting a body part because your enemy is generally an experienced and dangerous combatant who will resist such efforts. If you want to crush their testicles, you first need to beat their resistance out of them by removing their HP.
If you want to crush testicles with a magical hand, Bigby's hand can do it. That's a fifth level spell that does 4d8 damage, enough to crush most low level enemies and their testicles.
Clenched Fist. The hand strikes one creature or object within 5 feet
of it. Make a melee spell attack for the hand using your game
statistics. On a hit, the target takes 4d8 force damage.