I'm fine with deep lore being in something like Draconomicon, Glory of the Giants, etc., but for a MM I would have rather had more monsters than the "deep lore" sections we got in Volo's and Mord's - especially since for my homebrew, the background for the creatures they talked about followed older 2E lore and had gone a different direction of personal development, so it wasn't applicable for me.
For me, in the MM, a paragraph or two about the creature's role in the world is enough for me. As above, if I want something more in depth I'll go hunt down the lore specific books (Elminster's Ecologies, Draconomicon, Drow of the Underdark, etc.). A fair example would be the 2014 Aboleth entry. It's probably more than I'd expect (the bolded bullet point part and first sentence would do), but if I wanted deeper info I'd go search out the Aboleth section in 3.5E's Lords of Madness.