A mortal is something that has a soul separate from the body and can have an afterlife after death. Any outsider or undead is not mortal by definition. Neither are non-sentient creatures.
I am not sure about dragons - personally I would define them as mortals but in the lore they usually do not consider themselves mortals.
I don't believe there is a hard rule on this in CRB or Bestiary, but I think the upcoming Gods and Magic book is likely to talk about this, and there is probably already a definition in one of 1E books about faiths on Golarion that is still applicable in 2E as it's the same world.
EDIT: here it is.
Undead do in fact count as "mortal" if you'er talking about having a soul that has not yet been judged. They got diverted between death and judgment.
Yes, I was initially mistaken about the undead - it's a bit tricky to wrap my head around it, but by the rules they are indeed mortals.
In the balisse angel example though, their guardian angel ability works only on non-evil mortals and almost all undead are evil.
That section also specifies that Fey are mortals on the material plane but not in the First World.
Aberrations are just mutants and aliens- more of catch all for weird stuff, really. There are good aberrations- Flumphs are basically space jellyfish that come to say "Hey, you might not want to mess with that temple covered in C'thulhu carvings. You would probably not have a good time".
Animals, plants, fungi, and oozes fall into a similar kind of groove, they just have better defined biological similarities.
Giants are typically humanoids (same category as humans in the old system). Elementals, etherials, astrals, etc are outsiders I do believe (and we have already discussed their issues). Native outsiders, like tieflings, are mortal- most of those are just mortals whose bloodlines add a little something.
Undead are mostly evil and/or crazy. So the angel would not be very interested in protecting them usually.
A mortal is anything that will enter the river of souls when it dies.
If it's going to reform somewhere after it dies (like a fae or an outsider) it's not mortal. If there's nothing there to enter the river of souls (like a construct) it's not mortal.