Criticizing something that could be wrong with the methodology of a study is always 100% inappropriate. Either you've read the study and understand what it was trying to show and the methodology the authors used, in which case by all means demonstrate that and then state your criticism, or else you haven't and shouldn't comment.

But trying to get others to not read someone else's work or take it seriously based on something you dreamt up that could be wrong with the study design is a completely inappropriate.