If someone paints an excessively rosy picture of the past, what are some possible sarcastic or humorous responses in English? In my native language I could say something like 'sure, and the grass was greener', or 'the sun used to be brighter'. I wonder what phrases are used in English for this purpose.
I don't think there are any formulaic expressions There isn't a fixed thing to say. But your examples work well to express your sense (if paired with the right tone of voice and facial expression)
There is plenty of scope to be creative: "The sky was bluer", "candy tasted better". "Everyone had a pet unicorn". You can be coarse "farts smelled like roses".
Or you can speak without irony: "Dentistry was done without anaesthetic", "a third of children died before their first birthday"... and so on. Of course these are neither humourous or sarcastic.