Here the author is using understatement in saying 'a trifle'. What he really means is that it can be 'extremely' hot in Casablanca.
Sometimes we may say; "It's a trifle cold outside, take a jacket", when we really mean, "Put your jacket on, it's freezing outside".
"It's a trifle cold", "It's a mite too hot for me in Spain", "I think it rains a bit too much in Norway", "Can you turn the music down a little bit?"
These are all understatements where the speaker is playing down the seriousness of the situation.
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