I am a fan of custom monsters and the book really falls short in this deparment. It does have a section on it but it's sparse. I really like building my own monsters because my group has a habit of buying MM's and memorizing their stats so I like to keep em guessing.
There are similar problems in other areas of PHB/MM/DMG, that they could be put in one book or the other. For example, some people consider magic items to belong to the PHB because they see them as characters material, while others want them in the DMG. Traps and hazards are another example of something that could be both in a MM or a DMG.
So at the end it's really up to the publishers, and it's not only a matter of thinking where does a certain topic really belong, but also about managing the aggregate space of the 3 books. Mearls mentioned clearly that the original plan of the 5e MM was 320 pages, and they couldn't put everything in if they wanted large artwork, so they extended it to 350 pages. It's possible that they had thought at some point to put monster creation rules in the MM but then had to make room for more monsters.
I really don't have a strong opinion on where they belong better... but certainly since they decided to have them in the DMG, then the current (limited) info about monsters HD and proficiency bonuses in the MM is out of place, it should have been moved to the DMG as well.
I'm finding the actual stat blocks to be a bit lacking and they are really overusing giving monsters spellcasting. I don't always want to give everything spellcasting, sometimes I want to give them some unique ability or abilities and this book just doesn't provide that.
Personally I think the artwork could have been smaller (even tho artwork is one of my main reasons to buy a MM) in order to add more "crunch" to monsters, and the best would have been to add
optional crunch (like optional additional abilities, with an indication about CR and XP change) like many monsters had during playtest.
It's OK that they decided that the starting/basic version of each monster is simpler than 3e/4e, but then customizing them must become an integral (optional, of course) part of 5e, just like the same thing happens for player characters. While the DMG might have the
rule system for customizing and creating from scratch, the MM could have at least offer some immediate ready-made options. Really, we had lots of examples of "monsters variants" during playtest... what happened to those? Aren't they still there in the MM or have they been removed?