The planet Mesklin, the setting for Mission of Gravity, is a highly oblate spheroid, with an equatorial diameter of 48,000 miles, and a pole-pole distance of about 20,000 miles. In the essay Clement mentions "The model I have of it is six inches in diameter and not quite two and a half thick", so calling it a pancake planet is certainly justified.
Clement was clearly on good terms with Asimov; later in the essay he described how they spent an evening together debating what the major liquid phase on the planet would be, eventually settling on methane. Between friends a bit of teasing is not unexpected, and the joke title "Pancake in the Sky" is clearly intended to echo the title of Asimov's novel "Pebble in the Sky", published four years earlier in 1950.