I would say retrieving an information with no further action is ok.
An NFS mount, however, uses the system resources of the server and
is de facto usage of a foreign resource. There are table entries
made, to start at the least. When thousands of people in the network
would mount a file system, this can be a resource hog for the server
Looking what is there might be just ok, but if nothing is explicitely made
public (e.g. by naming although even this is very much on the limits), I
would not mount it.
The point that a public export is bad practice does not make it
legal to exploit it. When I leave my car running at 30 below while
in the supermarket, it is still illegal for somebody to take it ( of
course I would get nothing from the insurance, but this does
not condone the theft in itself).