We have to start with a definition of juxtapose in its literary sense. The Oxford English Dictionary does not include the idea of dissimilarity in its definition. It the derivation as the cognate French juxtaposer, giving the literal meaning of placing (Mediaeval Latin positio) beside (Latin iuxta)
Literarydevices.net offers a good definition as follows.
“Juxtaposition is a literary technique in which two or more ideas, places, characters, and their actions are placed side by side in a narrative or a poem, for the purpose of developing comparisons and contrasts.”
They give as one example a passage from Romeo and Juliet rich in juxtaposition, including both comparison and contrast
O, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
Like a rich jewel in an Ethiope’s ear;”
Shakespeare compares two similar things: Juliette’s shining beauty and the bright lights of the ball. He puts the brightness of her beauty standing out in contrast with the metaphorical cheek of (dark) night beside the equally contrasted brightness of a rich jewel against the (equally dark) Ethiope’s ear.
There is no doubting that the ‘juxtaposition’ in the quotation of the question is one of contrast, or, rather, of incongruity. What is said to be incongruous is the idea that Clem is strong-willed, set against the idea of his putting up with being made to dress as a chicken. From what the questioner has given us, it looks more like good-humoured fun than a mocking humiliation. So the word “forced” is unlikely to be meant literally.
There is a way in which this use is ‘stretched’. The author is not juxtaposing ‘two or more objects’. S/he is juxtaposing people’ experience of Clem’s strong will with (presumably) the weakness of Clem’s allowing himself to be made to make a fool of himself.
Does this involve a grammatical mistake? In a way it does: as has been said, it does describe a juxtaposition of A with B. But I am tempted to say that the word “juxtaposition” is redundant as well as clumsy. At best the reader is forced to go back over the sentence to check.
... the sight of someone as strong-willed as you being forced ...