As someone who lives within walking distance of the Horniman, it is absolutely worth it with or without the natural history galleries - if you are interested in fish, there are plenty of rarities in the aquarium (for a full list, I recently updated the one on Zootierliste, so it should be accurate), and the live coral displays are lovely. The cultural galleries are excellent as well, the gardens are lovely and the new temporary exhibition on dinosaurs is brilliant too, although admittedly a little small. It is certainly not a full day visit, but can easily be combined with Crystal Palace Park, which you mention you intend to visit for the dinosaurs. If you are limited to public transportation, I would recommend taking the 363 bus between the two, as its destination at 'Crystal Palace Parade' would allow you to pass the remains of the Crystal Palace, the sphinx statues, the stadium and the original water fountains on your way to the dinosaurs and the farm.
Personally, I would love to see an outsider's perspective on the collections that I am familiar with, and doubt that I am in the minority here. If you get the time to write such a thing, then I think it would be an excellent read indeed.
Enjoy your trip!