You can chain callbacks in a Drupal 8 migration

If you're wanting to do some advanced processing (e.g. lowercasing a word and then capitalizing the first letter) you can chain the two callbacks to get the desired effect:

process:
  name:
    -
      plugin: callback
      source: MyVar
      callable:
        - '\\Drupal\\Component\\Utility\\Unicode'
        - 'strtolower'
    -
      plugin: callback
      callable:
        - '\\Drupal\\Component\\Utility\\Unicode'
        - 'ucfirst'

The two transformations will take place one after the other. The first one will execute and pass in the value straight from the source (e.g. 'SOMEVAR'). The next one will get the result (e.g. 'somevar') and then the the final value will be ('Somevar').