A little boney is normally okay. What I mean by this is in a guinea pig that is a healthy weight you should be able to feel the bones of the spine, shoulder blades and ribs (in guinea pigs the ribs are the easiest to feel, depending on their build it can be very hard to make out the shoulder blades), but there should be a healthy layer of padding between the skin and bones (so you can feel the bones are there, but couldn't make out every single ridge of the spine, or you can feel where the shoulder blades start but not where they end).
If an animal is underweight there will be very little padding between the skin and bones so it will almost feel like you are directly touching bone when you feel them down, you can make out each and every rib, but they may still have a little bit of padding between the ribs. Severely underweight guinea pigs will distinctly feel like nothing but bone and skin around the spine and ribs and shoulder blades, the skin will sink into the gaps between the ribs, and the shoulder blades will sharply protrude.
This also depends on the age of the animal, guinea pigs do lose muscle mass as they age leading to a more boney feel. Also female guinea pigs suffering from ovarian cysts can, along with other symptoms, have a redistribution of weight causing bony shoulders and a round abdomen (see
https://www.guinealynx.info/ovarian_cysts.html ).