If the museum has socks for centaurs, it should have wheelchairs for mimics.
Eh... centaurs are a fairly 'common' species, whereas mimics are less so (insofar as in the previous comic the train attendant couldn't identify her, whereas there have been multiple centaurs in the series). So specialized equipment is probably not available. And while art gallery would probably have some dollies or a hand cart that could be used, the fact that the mimic-girl has already caused two separate disturbances (while the most recent creating a hazard at that) they likely just want her out.
Serious accessibility issues, though! Clearly the wheelchairs/dollies/hand cart aren't clearly labeled or immediately visible, if there are any — and how exactly is she meant to get them if she can't move inside the building without causing a disturbance?
I mean, people who need wheelchairs usually bring their own, no? The gallery probably has a wheelchair ramp, but while socks can be one-size-fits-all, wheelchairs kinda have to be fitted to their users anyways.