Of course it was appropriate to laugh and smile during the sermon. The preacher was laughing and joking. He was trying to be engaging and amusing "let's get y'all married".
In a religious wedding, the solemn moment of a wedding is the marriage, when the royal family were appropriately sombre and sitting with heads bowed etc (couple making deeply personal, solemn, covenant, promises in the sight of God). The sermon is then an address to the couple and the congregation, its a "speech", and completely appropriate to laugh, smile, respond. And 17 minutes? For a sermon, in a church, with the ABC and Queen present.... Absolutely fine! It is a religious ceremony after all.
Feel like people just desperate to find fault. RF look solemn and they are "not interested", RF smile and laugh and they look "disrespectful". Also, if you are sitting in the choir stalls on the same side as the bride and groom you can probably see precisely nothing of them. So guests might have been listening intently, but pretty hard to gaze at them adoringly!