I feel like the tone shift was actually very similar to Mulan's, they were still joking around until they reached the village and saw the carnage, then we get mostly serious until now
First of all you need to know when to actually switch to the humor from the seriousness. Just like with jokes in general it's all about timing. The fact that the previous chapter was entirely dedicated to a very serious moment of a woman dying and thinking about her life to then immediately open up this chapter to a guy's ass being groped is not good timing.
I mean, this feels more like the story was mostly from the second royal guard pov since the fight started and now its from everyone else's, before the death felt impactful and meaningful, it was someone dear to them that died and to these ppl not used to war it was unbearable, now we're seeing things as they are, this campain was a resounding success, only one death was a miracle and in everyone else's eyes, they should be celebrating, not crying their eyes out for one measly person
That one death was so meaningless to Astarte that groping his ass was far more important than even going to taunt her sister
Hell, even the beginning feels like its for a change of pov, we're following Helga, all the crying? All the sadness? thats all background noise, nothing to do with her, all that matters is her lord