What are you trying to accomplish in the end?
There are two places that the frame rate matters: recording and playback.
By default, iMovie will export a movie at 30 frames per second. Whatever you are watching in your iMovie timeline, regardless of source frame rate, will play back "at real time" at 30 frames per second. So if you recorded 120fps for one minute, import that in to iMovie, then export, you will end up with a 1 minute movie playing at 30 frames per second.
But the extra frames allow you to do slow motion, while retaining the "smoothness". Again, if you recorded for one minute at 120 frames per second, you can slow that video down to 1/4 so that it is playing "slow-mo" over the course of four minutes, but playing smoothly at 30 frames per second.
If you started with a 30 frames per second video for one minute, and slowed it down to 1/4 so it takes four minutes to play back, you would end up with a fairly "jerky" video, playing at 7.5 frames per second (even though the iMovie-exported "container" would claim to be 30 frames per second - it would play each recorded frame 4 times in a row.)
Summary: If your goal is to turn parts of your video in to "slow-mo," then record at the higher frame rate, and don't worry about what you're exporting.
If your goal is to have "smoother playback" (playing 1 minute of 120 fps video in 1 minute at 120 fps,) then, as rsgaming123 mentions, you can export at faster framerates, but be aware that many devices don't like to play back those higher framerates.