At the moment SegWit2x forks, will mining be way easier? Is there a need for people to run full nodes that are mining on CPUs when/if the SegWit2x fork occurs?

Will almost all of the mining power be switched over to SegWit2x?
No, it will not be easier. The "difficulty" of mining is determined by the difficulty (the value). The difficulty only adjust itself every 2016 blocks, so when the Segwit2x fork happens, the difficulty will not adjust until 1152 after the fork happens. So the difficulty will not change at the time of the fork for either chain (barring any difficulty adjustment thing that segwit2x decides to implement in the future) so it will be equally difficult to mine on both chains and thus not easier at all.

When the difficulty adjusts, it can only adjust by a factor of 4 at most, so only down to a difficulty 1/4th that of the difficulty at the time of fork. This means that it won't be possible to CPU or GPU mine either chain for a while as the difficulty needs to adjust down a lot.