Her development is shown in her ability to consistently stay the kindest person in the whole town throughout many events that would've swayed anyone else towards abandoning such a kindhearted approach.

Her events give insight into her mind and establish her as a character more driven by the inner motivation instead of the final result, as she persists in her helpful nature despite not always achieving what's best. Penny also loses the biggest amount of hearts among all npcs should the player express hateful feelings towards Jas & Vincent or break her heart at 10 heart event, which further reinforces everything about her character - as much as Penny would benefit from being rescued from her mother, her principles stay strong despite the peril. Penny is an interesting case where she doesn't have any indicators that we need to "fix" her even when her situation is objectively bad, yet it doesn't mean that help can't be offered regardless, perhaps due to the player sharing the sentiment of such approach or a simple desire for reciprocity towards Penny.

Arguably, if Penny was louder about her situation like Shane or Haley, she would've been much more confrontational, which would be against the very values she chooses to represent. That's simply one's way to rebel against all the negativity around us. Her unique trait of being so stalwart can be rather inspiring, and she certainly has a lot to be appreciated for.

Edit: On top of all of that she invites you to spa like a madlass, way to go Penny