My mother is cheery, but very repetitive. It gets a bit wearing at times but I just ring the changes every time a subject comes up again (usually at 3-5 minute intervals).
So I am grateful she is not grumpy like Minimo's.
Can you just keep replying with a positive answer? Even if it's made up, or not quite true, and gets a bit tedious and can be hard work.
Her: "Raining again" - You "but bright sunshine predicted tomorrow (even if it's not what was forecast)
- I love the autumn sunlight, and the beautiful sunsets"
Her: "Dinner didn't taste very nice" - You: "Next time we'll try adding some (spices, salt, sugar, peas)? Or - "It's my favourite, but I take it with a pinch of (cinnamon, knob of butter, spoonful of honey...)?
Her: "My back hurts" - You "Can you stretch your toes out and wriggle them?" Distracting her might help her forget it and making her move slightly might alleviate the pain a bit.