9 months post op hair transplant & lacking density, is this a fail? (Photos)

Without knowing how many grafts were placed it's difficult to answer your question. However:
1. I don't ever evaluate for final result until the 12 month point. You may have further growth coming
2. There will ALWAYS be some "density mismatch" between grafted areas and native hair.
3. In the area you appear to have had transplanted, I would say 1200 grafts would have been the estimate in my experience to get reasonable density, maybe closer to 1400. I hope this helps.