The 1888 LDS large-print edition switched the word order in this passage so that the negative not came after the subject this rather than before it. Elsewhere there are four examples of the word order “is not this” but none of “is this not”. In fact, one of these examples occurs later in this same verse: “and is not this our afflictions great” (Mosiah 7:23). This second instance was left unchanged in the 1888 edition, so the change in the first instance was probably unintended.
Summary: Maintain the word order “is not this” both times in Mosiah 7:23 (and elsewhere in the text); the alternative word order, “is this not”, occurs nowhere in the text.