The expression "implemented on top of" is associated with the notion of
platform, see
http://en.wikipedia.org/wiki/Platform_%28computing%29[
^].
I don't think "on top of a cloud" is a widely used term. It would be, again, more correctly to refer to a platform. Consider this overview of
cloud computing here:
http://en.wikipedia.org/wiki/Cloud_computing[
^]. Take a look at generalized cloud architecture and locate the "Platform" layer on a picture and the section "3.3. Platform". And, on top of a platform…
—SA