I think it is okay. We have enough critics as it is. ;)
I think most do not comment for fear that said marketer or creative agency may one day be their employer.
A rule (in my opinion) that I try to follow, as is the case with any critical critique of creative; if you have a complaint or issue with the work, be sure that your critique is part of a solution. Let it be well thought out, not impulsive and arbitrary. If you do not like it and can not formulate a reason why, it's best to not convolute the information used to shape the work. Ultimately, this undermines the talent and help of everyone involved.