No, ET (or E.T.) is not an acronym by most all definitions (although it is an abbreviation). An acronym is formed from the initial letters of several words of a phrase. There is some dispute as to whether to qualify as an acronym, the set of letters has to be able to be pronounced phonetically (like radar) or can be pronounced by pronouncing the letters (like IBM).
In the case of ET, it is an abbreviation of extraterrestrial, a single word, so an acronym of it would be only the single letter E. If ET were an acronym, it would stand for extra terrestrial (which if it were understood, would mean something like "an unneeded earth being").