Kris is one of those lads who has never been fortunate enough to gain a good education. Otherwise he would have known that 'pleb' is not a word that could ever be used with regard to Freddie. Freddie is quite the opposite of a pleb - an uncultured person of the lower orders - in that he is an Oxford graduate, lives in a stately home, and is a wealthy upper class lad who wears his class education and wealth very well.
Kris thinks that having a head of Shirley Temple curls gives him the right to criticise someone with brains. In fact, to make a statement like he did, it is obvious that under those Shirley Temple curls throbs a grey-matter free space.