I've never had a problem with the food at the Horniman Cafe - but then you can't go far wrong with a cup of tea, cartons of drink, muffins and ice creams. I wouldn't have thought it would be a venue for a main meal and this may be where it is perceived to be going wrong. It should stick to cafe-type food - cakes, sandwiches, maybe salads, maybe paninis etc. but not go for hot meals.
As regards price, I'd say it compares very favourably with the sort of prices you might pay in town and I don't begrudge the Horniman a bit of crosss-subsidy, as it is a great FREE museum!