ok, so this is more like the Linda Problem. It is more likely that a person is a shitty person than a person is a shitty person and has a mental illness.

Hmm, there's an element you've left out of the situation that makes it different from a straight Linda Problem though. In logical terms, it looks like this.

  mental_illness -> shittiness
  being_a_naturally_shitty_person -> shittiness

And we know know two things: 1) the person in question is exhibiting shittiness 2) the person in question has been diagnosed with a mental illness. So:

  mental_illness = true
  shittiness = true

Whether being_a_naturally_shitty_person is true or false is something we have no information on.

So, from that starting point, we already have an account for the shittiness being exhibited since:

  mental_illness -> shittiness

  and

  mental_ilness = true

So if we want to go ahead and make the additional claim that being_a_naturally_shitty_person = true —you're violating Occam's Razor by introducing an unnecessary second cause when we already had a sufficient first one.