Well, I think every individual who reads a recommendation letter can interpret it differently depending on what the criticism is. Personally, I won't hold it against them if there was some mild criticism (late for class a couple of times, minor mistakes, or such, as long as the letter says that the applicant has progressed) - but that's just me.
I know that everyone's human and unless the applicant has a GPA of 4.0 and an MCAT of 45, I would be slightly suspicious if a letter has nothing to say in terms of constructive criticisms or says that the applicant is "perfect".
To me, a letter that sounds too good to be true, or is too vague to describe why the applicant is "perfect" either means that the professor doesn't know the applicant well enough and is writing a generic "he's so great, you should accept him" type recommendation letter. Obviously, have a glowing recommendation letter from a professor that knows you well and that can explain why you're a "perfect" applicant is better than one that has mild criticisms, but overall - I don't think there's any problems if there is some mild criticisms so long as you've progressed or worked on improving those aspects.