Except this time, it actually meant something. In the past, Ash was only kissed by someone he just met, as a sign of appreciation. However, with Serena, this is the first time Ash was actually kissed by someone who loved him.

Serena had a crush on Ash throughout the entire Kalos saga, and she was also the only one of Ash's companions to have her crush as a repeated occurrence in the series, so the implication that she finally kissed Ash before saying goodbye, really makes it as a more meaningful kiss.

I'm also guessing she kissed him on the lips, where in the past, Ash was only kissed on the cheek, just look at Clemont, Bonnie, and Pikachu's reactions. There's no way they're giving shocked reactions over seeing Ash getting kissed on the cheek.

Too bad none of this shit is gonna matter, be referenced or even acknowledged in the future. All this development for nothing. Does his mom even exist anymore?