From what I recall, he's not always supposed to. Indeed, there will be times where he does nothing at all. Trinket pets, in general, are pretty useless.
I am thinking the prob may be in spaces where the ceiling is low as he drops a rock on enemies for a burst of dmg. It looks like it large spaces the rock drops and in small spaces it doesnt.
Not true. One of the ones I got for the summer event, the little lobster looking guy, did a good job at aggroing enemies.
The Boo's are pretty useful, the pet from the aquaman event is really usefull for trolls and can be used rotating ( if you don't have a boo) to do more damage, the jack pet from the halloween event can be used by healers too, rotating with another trinkets. I'm sure you didnt mean this ones, but sidekicks and back ups are useful .
This is the same as last year's thread tbh; some work and some don't. one of my officers has a lep pet that works.. i have one that doesn't. perhaps its a either a bug or 'in-joke' by devs. who knows?