Unlike fake 'cures' and many other things that will have a negative and potentially deadly impact, I don't really see the downside to showing unity and solidarity. Many people cannot afford to donate money and many who clapped already do. They are not in a position to pay NHS staff more, but they can show support, from their own homes, which is also a way they can help.
If the side affect is that some people feel smug when the shouldn't then I don't really care, it's bigger than that. They wouldn't have changed their whole outlook without a round of applause.