Circus acts amaze us because they show us familiar things (people or animals, these days, pretty much exclusively people) doing extraordinary things. When a human accomplishes what a monkey can do naturally, we find it amazing, when a monkey accomplishes some intelligence task we only attribute to humans, we find that amazing. We find it impressive that a lion does not bite off the head of the man who has stuck his head inside because the lion is a wild animal and difficult to train.
In an intergalactic Circus, there will be acts from all over the galaxy, with many different and exotic alien lifeforms.
People from individual planets do not travel off world very much (and most never get off world their entire lives) They know of, but are not familiar with most of the alien species in the circus. They have never had direct, personal contact or have made acquaintance with any of them, or their kinds.
They know of other aliens mostly via third parties e.g. television. But there isn't much about them. The level of unfamiliarity equals 18th-19th Century Europe first encountering pygmy peoples.
Given this, how can lifeforms of one kind experience and understand, viscerally, feats of another species/ alien?
Are that acts that can always amaze throughout the galaxy, despite possibly huge differences in alien physiology?
Won't it be so that what may be impressive when seen done by humans could be absolutely mundane when observed being done by another species? (e.g. human trapeze artist vs mokey trapeze artist...) So, apart from the circus being a 'zoo' of exotic lifeforms, how would it impress and awe?
Assume all aliens the circus will perform to have evolved on an Earth-like planet. Also that the performers will have to perform on actual Earth, to humans, at some point with limited life-support systems. They are limited by the laws of physics, most likely breathe oxygen, they are mortal, and they can understand (maybe even empathise) with human emotions.