Yes, the majority of what they serve appears to be reheated by microwave. When they tendered out to the folk running the cafe the museum managers clearly did not make a decision based on the quality of the food they would serve or they'd surely have sued. I suspect in a rare moment of disregard for attention to detail they put profit ahead of their public.
Give them their due though the staff are incredibly child-tolerant and the place is always clean.
The food is equivalent in price and quality to the average motorway service station. It is not befitting and it's nice to know I'm not alone in thinking it. If there's a consensus view on this thread I'll email the Horniman with a link.