Jeans and jumper? Never have I ever been to any formal occasion (christenings included) where people show up in the above getup. Thankfully. You can scruff around literally any day of your life, why would anyone choose to do this on a celebratory occasion. Would it kill people to put some effort and display some self respect once in a while. But like I said, haven't even seen anyone wearing hoodies during christenings.
That said, the dress is a bit too evening-y, IMO. Nothing wrong with it, a conservative length and colour, but bodycon fit and lace would be better for an evening, maybe. Something ligther and A-line/tunic/slightly loser fit would work better (just not 'jeans and jumper' ffs).
And to that one 'christenings are not about what people are wearing, it's not a fashion show'. Christenings are not ashes on the head and burlap tunics, it's a family celebration. Usually followed by a formal dinner, get together of the guests or a party. And people dress up for those. Many people plan their outfits and don't just throw on the first thing they find beside their bed. And there's nothing wrong with that.
Following that logic, you can rock up to the wedding in trackies, it's a 'union before god' or some such bullshit (I'm an atheist), so what does it matter what people wear, god has eyes for everyone, no? And no need for any party, better to donate all the money to the poor, right?