Speaking for myself, I don’t mind when people add comments or opinions to my gifsets. Even if that’s critical or negative – as long as it isn’t bigoted. (I’ve blocked my fair share of racists and homophobes.)
Often, the visuals of gifs are a good springboard for people’s commentary about a piece of media or a storyline. I’ve always personally considered it a natural branch of fandom engagement on tumblr. Occasionally it’s even interesting to read, even if I vehemently disagree.
That said, I certainly respect other giffers and creators who feel otherwise, who dislike unsolicited negative opinions. I understand why they do, especially in fandoms where personal attacks and vicious opposition are commonplace. And people can be entitled. Either way their wishes should be respected.
I just wanted to “new post/text” my personal feelings on the matter, to let people know they can pour their heart out on gifsets I make.