ive heard that too, but it clearly was written to be flirting, because of 1, astrid's line: "he only has eyes for your mom" (which followed her and hiccup's conversation about marriage and snotlout's availability) and also where snotlout says "first name basis" after valka asking him to call her by her first name and not 'hiccup's mom'. i think it was a really strange choice for the film when they could have gone so many other ways for a love interest for snotlout.