I worked with a number of different scanlation groups as a proofreader in the past and one thing that made it enjoyable was adding the occasional comment or joke that would get put into the margins of pages. But then, I usually worked with groups that also enjoyed doing this. So I heartily approve of comments by scanlators, whether they are kept to the credit pages or inserted into the margin spaces of chapter pages. If it helps make the actual hard work of scanlation more enjoyable to those who do this as a hobby, I don't see why anybody should complain.
But to those who do complain, I can only offer this advice: They should either wait for an officially licensed translated version to be published and buy the volumes with their own money...
...Or their complaints can be answered by that famous quote from the Divine Miss M, AKA Bette Midler, who so eloquently said,
"FUCK EM' IF THEY CAN'T TAKE A JOKE!"