If allergies are not a concern, consider adding ground nuts and seeds to the yogurt she will eat or any other smooth consistency (applesauce?). Oatmeal works too. My 17mo DS wouldn't eat when we started foods about nine months and his weight gain was poor. We eventually began adding ground nuts and seeds to yogurt (or anything!). We started with tahini and progressed to pumpkin seeds, almonds, flax seeds, etc. [Also tried wheat germ.] This habit extended to the rest of the family and I try to remember to add these powerhouses of protein, fiber, healthy fat and more to everyone's meals at least once a day. I used guidelines from Super Baby Food (by Ruth Yaron) and Disease Proof Your Child (I know, awful title) by Joel Fuhrman. Yaron's book lists how much fat in a 1 oz serving (e.g. 15g in an ounce of almonds; 13 in an ounce of pumpkin seeds!) and this was helpful. Note that you can roll bites of "wet" fruit pieces (bite of peach, banana, etc) in finely ground nuts/seeds if your daughter will accept the slight change in taste/texture. Makes the pieces easier to pick up, too. I often sprinkle them on top of cut fruit before offering to my son. Agree with PP that avocado was a huge fat source for DD although DS only recently started accepting it as "dip" on a cracker or pita.
Just a reminder with the nuts/seeds that they can go rancid quickly. Some folks recommend grinding only what you'll use immediately while others support freezing. I buy small amounts of bulk, organic seeds/nuts and keep them in the freezer. I grind a few days' worth and keep this in the freezer as well. Best wishes from another tall and thin mama trying to get healthy fats into her skinny kids!!