Chara does not find the Amalgamates existence to be “humorous" 

Despite what people may interpret this bit of narration when you select "Laugh” to mean.

If you ignore the “it’s not funny” callback to Snowdrake’s dad that the flavor text makes when you select laugh twice: (link) (link)
Or if you think that the "laughing to hide your sorrow" explanation is something that has no in game context to support it: look at the truth mimic bug for evidence that it does (link)

Then yes, you would probably read this at face value and walk away thinking Chara legitimately thinks that the Amalgamate’s condition is something hilarious.

But the thing is…they don’t.

The above is what comes up after you Check Reaper Bird when you are able to Spare it. What was once an indecipherable message now is an observation full of hope for the future.

All the Amalgamate encounters start off terrifying and are handled with an uncertain caution by the narration (and rightfully so, these battles are dangerous and ramp up the difficulty in this final segment of the game) and in the case of Lemon Bread maybe even with fear? (Call, Scream, and Cry are all actions that do nor relate to any of the monsters in the amalgam but are made available.)

Snowy’s mom may have the “Laugh” and “Heckle" option but why wouldn’t she, when the previous monster encounter that relates with her monster type (snowdrake, her son) also had those options? Except this time the way to spare her is with the "Joke” action.

You could try to argue that it’s the state of her body that is making Chara laugh but there is another monster who can be seen in a similar state to the Amalgam that Chara shows no signs of enjoyment in.

In Undyne’s neutral death scene she starts to "loose itself" in a near similar fashion to Snowy’s mom, and here is how the narration reacts to it:

It’s a solemn incomplete observation that trails off into one of the few examples of the narration going completely silent. (link)

If Chara is the narrator we can not just cherry pick bits of narration/flavor text and develop their entire psychological profile from there. We have to look at all the narration, cross examining the differences and the context of the situation, in order to form a solid idea as to what kind of person Chara was like, is like, and can be like.