This is based on my opinion and my experience with MAL
Mentioning for the sake of context: One of the main function of MAL is a platform for anime viewers to come together to share their knowledge, reviews, opinions, recommendations etc., and also to keep track of a list of animes that they've watched. The site doesn't do anything by itself, as the rankings are community-driven. Thus, lets say for an anime, 4 people ranked it 10, and 1 ranked it 1, it would come to an average of 8.2 (In mathematical context, this should be a subjective answer as this average doesn't really makes much sense, it should be something like an outlier, thus, for this case, using median would be more accurate to say that most people think this anime is a masterpiece).
Anime scoring & individuals taste:
Now, as @кяαzєя♦ mentioned, everyone has their own taste in anime. Thus, there will naturally be some people giving high scores to certain anime, while others giving it average to low scores. However, this still doesn't really answer the question very clearly. If you notice, why are some of the worst animes out there still greater than a score of 5? (The middle point between 1 (worst) and 10 (best))
Different definition of "average" ranked anime:
As an example, personally, I'm one of the MAL'ers that will give the LOWEST ranking to (IMO) WORST anime a 5. The only reason is because (Once again, IMO) I personally feel that to me, despite the mathematical context, 7 is the "average" ranking, 5 is the "worst" ranking, and "10" is the best ranking. The main reason is because when I watch something, and I think "Meh, it's alright", giving it a 5 makes me feel very uncomfortable, like it still has so much more ways to go (6 -> 7 -> 8 -> 9 -> 10). Thus, I re-scaled my own ranking for this, and adjusted it accordingly to my preference.
Anime names and its inconsistency to its content/plot
Honestly, I think the example you gave is actually intended by the original author/producer (I have not done research in this. However, just taking a look in the reviews of that anime, I interpreted it as it is a titled made to be misleading intentionally).
Thus, at least for the example you provided, it shouldn't be MAL's fault.
IMO, MAL does a good job at what it is, a platform for anime viewers to interact and manage their list. However, if you're going to take recommendations, I still think MAL is a good source, because as a community, most (but not all) the recommendations are still pretty good. However, knowing that everyone has different taste in anime, every recommendation/review should be taken with a grain of salt.
(Though, of course, you should just watch what you like. MAL just helps you by providing the potential titles you may like)