Soma has never been the underdog in the traditional sense, general perception in story skew against him, but we know from chapter two when he impressed Erina with his cooking that his talents were world class.
When he's in difficulties he overcomes them, when he loses he learns from them.
Even without the "shonen formula" soma exhudes victory because of the way that he deals with the challenges and defeats. Even when he losses it feels like he could have won.
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I think that it has to do with the fact that cooking is a lot less violent and damaging, and the measuring is subjective, soma can't suffer during battle because battle is comfortable and it is always decided in the end, and without that ingredient, without the other components of an underdog, then he can't work as one.
Compare to Deku or class F. Or even Luffy back in Alabasta, during croc round one.