Under the level system, energy and mood were always the same, but we don't have a good reason to think they will be the same under the character-upgrade-by-imagination system. The fact that they were the same could be seen as an accident until now, for a somewhat anachronistic but personally-beloved meaning of "accident."
I already made a post talking about upgrading our energy, which Scarlet Bearsdale pointed out was based on the possibly faulty assumption of a connection between quoins size and energy. We'll see how that pans out. But what about upgrading our mood pool?
It's very safe to say that given the current mechanics of the game a larger mood pool is unequivocally a bad thing. There is no upside. The amount of mood you need to get your xp (soon to be imagination) bonus is based on your maximum mood. The amount of mood you lose over time or by poisoning trees is based on your maximum mood. Nothing beneficial is based on your maximum mood.
If we could lower our mood pools to 10 then we'd just have to sniff a butterfly milk and we'd be getting maximum possible xp again, even if we'd just come back from the dead. Or we could toss betwewen 1 and 10 grain into the nearest shrine to top up if we were somewhere in between. Realistically the optimal amount of mood would be 149 - the precise amount where the mood loss per 60 seconds rounds down to 1.
There could definitely be changes that make mood work differently, but until we see what those changes are, I strongly advise against increasing your mood pool at all.