I'm not saying that he will be "put off" just that it might not be as big an advantage as you might think. All the top riders will be reasonably well prepared for riding on cobbles, and it's actually less technical than a race like Strade Bianche when there are descents. Again Colbrelli won the last rainy edition and he had no background in off-road stuff. MVDP will probably win because he has the strongest legs and not because of his CX/MTB background. But, if anything, having rain randomizes it and increases the chances of an upsetIt's possible, although autumn 2021 was probably the low point of Mathieu's form and health struggles of recent years. Returning from his Olympics crash his road results were pretty poor, and following Roubaix his cross season lasted all of 1.5 races.
I don't think Mathieu will be put off by the rain. Cold and rain, perhaps, but the forecast does not look particularly cold for the weekend.