I don't like the My Little Pony games aimed at 6 year old girls, but I won't say they are bad because I know I am not a 6 year old girl.
It's like Solitaire. I've played countless games over the years, but I don't recall every actually enjoying the time spent playing it. Normally I was just avoiding doing something else.
PopCap are definitely the company to look at here. They spent a decent number of years cloning smaller games, which was not to their credit. In the last 4 years or so, they've been creating amazing original IP. And those games are doing gangbusters with all sorts of audiences. You don't have to create bad games in order to engage new audiences, you just have to tap into what they are interested in.
Self-assessment isn't a perfect metric either, but differences in self-assessment between players of different kinds of games might give something interesting. I mean, if you go only by money made, the slot machine is still the best game of all time. Which maybe is true on one axis of "best", but it seems weird to argue that it's the only possible way to discuss games. Is looking at the top-grossing films list the only possible way to discuss films?
Of course you can make up metrics based on averages of everyone's opinions, or weighted averages where you weight by the "sophistication" (in some sense) of the person's opinion. But nobody will believe such metrics anyway, in that you'll never catch anybody saying "I hate this game/book/album, but this reliable metric says it's good, so I guess it must actually be good!"
If Farmville is bad, it falls into the "fascinatingly bad" category of things that are derided by professional opinion-makers but still vastly popular. Genuinely bad games, books and bands are a dime a dozen -- check out a publisher's reject pile, an open mike night or a twelve year old programmer's hard disk for examples -- but fascinatingly bad things like The Da Vinci Code or Nickelback or Farmville deserve a lot more analysis.
Game reviews do not use a good game checklist to explain why a game is good or bad, they give their opinion. You can say Zynga games are bad, but supposing that your opinion should apply to everyone is arrogant.
By what metric are other art forms judged? How are the Oscars awarded to movies? Who chooses the winner of the Booker prize? Or the VMAs? Peer approval.
Zynga doesn't have peer approval from many game designers. This article highlights some dissenting opinion, but there's plenty more out there. Just Google it. Their infamous speech speech at GDC Awards show doesn't help their cause either. [1]
Note I am careful not to say "games reviewers" because reviewers are writing for their audience, and that core game audience usually does not intersect with Zynga.