They've been doing this for at least a couple of years in relation to the pre-flight ground experience at Heathrow. Albeit text message based rather than app based (with the option to complete the survey via a link too). Although I noticed that the text messages seem to stop around the time the FLY hit the fan.

Personally, I think it works better for the pre-flight experiences, since usually people have more free time then. Whereas post flight I'm going to be less inclined to respond immediately, as others have said at that stage you want to get from A to B, check emails, etc. Responding to a survey is lower on my priorities at that point.