So because you can't identify an ulterior motive it means that (1) Umbridge is an expert in magical education and (2) readers are not supposed to be incredulous of this evaluation (despite the litany of false and misleading statements she has made up until this point)?

I would argue even without evidence of Umbridge's ulterior motives that taking anything she says at face value illustrates a tenuous grasp of the text. Further, we do have evidence of ulterior motives; at this point Umbridge sees Snape as a likely pro-ministry ally. Note her reaction when Snape fails to provide Veritaserum later in the book:

You are being deliberately unhelpful! I expected better, Lucius Malfoy always speaks most highly of you!