It's pretty hard to create a vote/approval system for recommendations. For instance recommending a manga because the main heroine is a maid in both, is perfectly valid. Likewise, recommendations based on a theme or context is ok too, even if the feel of the manga is different. As long as you justify it like on MAL, if it is clear why you recommend it.
The thing is, once you ask the community whether the mangas are indeed similar, they will not choose whether the recommendation is valid (if there is indeed a common point), but on whether they, themselves, would recommend it. This is different. If what I like in the manga is the drawing style (or the storyline, or the theme, or the characters, or the "feeling", ... ), any recommendation made upon a reason I am not sensible to for this manga will probably be downvoted by me. I mean, for me, appreciating this one will probably not mean appreciating the other, as they are pretty different in what is important to me.
That's why I think recommendations approval is difficult, because people are generally not appreciating manga the same way, for the same reasons.
