Hola Thea

In this sentence we are talking about something hypothetical:

This is what triggers the subjunctive. So here the speaker is saying "the less I see you [from now on, in the future], the better"

However, we can make a similar sentence with the indicative exactly with the same structure but indicating something that actually happens (not hypothetical):

Here, her seeing him less is a fact, not a future/hypothetical idea.

I hope this clarified it.

Saludos

Inma