How about setting the fontSize value to a string - "1rem" or percentage "77%", or even from the MUI docs try setting with media-queries, kind of extedning what you've began:

theme.typography = {
  fontSize: '1.2rem',
  '@media (min-width:600px)': {
    fontSize: '1.5rem',
  },
  [theme.breakpoints('md')]: {
    fontSize: '2.4rem',
  },
};