Q:

Can I disappoint people? 

It seems like the villagers like every house I design so far.  Is that because they love me no matter what, or because I am actually skilled at putting houses together?  
Is it possible to disappoint someone and design a house that they aren't happy with?  

A:

Pretty much no, there are bare minimum requirements for many homes (the stuff they have in cardboard boxes) and facilities (general requirements that Isabelle won't let you fail to satisfy).  I made a couple joke homes that are impossible to navigate or contain a single item copied over and over and they treat it just as gleefully as any other home. 
There ARE certain items that will make a heart appear over a villager's head or make them clap, but this is just an indication that you're matching the theme.  They'll react in horror if you dispose of one of their "prised possessions" (even if you clone dozens of them and only toss one).  But the actual reaction once you finish the home is always positive best I can tell after a couple dozen trials.