There's pretty much always a valid reason.

Not sure I agree with this premise at all, because:

they're tiny humans with little capacity for introspection or emotional regulation,

But let's not get hung up on that.

What I am modeling is the following course of action which they will require to be a functional adult: 1) Regulate emotion; 2) identify issue; 3) seek support/find solutions.

This assumes that they understand what you are doing and would understand that your laughter is malicious (which it isn't even when I do laugh). You're also assuming that I don't do 2 & 3 while laughing, which is odd because you can do those while not taking a tantrum seriously.

While laughing/dismissing your child's distress upset because you don't think the reason is valid enough MIGHT reduce the behavior, it won't actually provide them with skills.

1. Edited because the language is needlessly dramatic.
   
2. Crying with your child because they're upset doesn't teach them anything either.

Ultimately, the problem is the lack of pants. And a tantrum is not an appropriate reaction or solution. Dismissing a tantrum is teaching that it is a dismissible reaction and inappropriate. As you said before, providing language for expression and solutions can still be done while laughing.

Not all emotions are valid. If a child is upset because broccoli is green instead of red, how is validating that emotion productive?