Is there a mod (or a way to mod, using s3pe) the changing table so that sims can use it anytime? I find it so annoying that sims can't use it when their mood is too low.

I have never had that happen when their mood is low and I'm curious as to what happens when you try to use it when mood is low. Or am I completely misunderstanding and they just do not use the table autonomously?

I am not aware of a mod specifically for this but there are settings in NRaas Retuner for the Changing Table.
From City Hall or a computer>NRaas>Settings>General>By Object>Changing Table.

When you try to use it when their mood is low, the interactions is greyed out, saying either that their mood is too low or that they are too tired/stressed/hungry/whatever.

Thank you AsaButler. I do recall seeing similar notices for other actions initiated but I don't recall ever seeing that with the changing table. Thanks so much for responding.