As a game developer who has published a casual game on BigFishGames.com I speak from experience when I say that the highest quality games bring in the greatest amount of money. In any given day, a number 1 game there will be bringing in 2x as much as the number 2 game. And a number 1 game will be making about 10x more than a game which is ranked 5-10.
So with that in mind, you would have to create 10 'average quality' games (by BFG standards) to make the same amount of money as one really high quality game. A high quality game will receive much higher profits than many low quality games which take the same amount of time to develop.
Big Fish Games has a minimum quality standard, such that they won't accept really poor games. This means that for your game to be accepted you have to have worked on it for quite a while to make sure it's polished and actually fun. So if you assume some sort of 'polish/making game fun' time overhead, then your time is better spent developing a high quality game, otherwise if you're polishing bad games then the amount of time taken to polish them won't be worth the return.
Having said this, it is however possible to have a series of lower quality games and make a good income. The catch is that you have to release them in appropriate places.
Every time you publish a game, it's another source of residual income (from continual sales). So as long as it's "sort of fun" then there will be people who will play it and you. If you release games independently from portals (such as on your own website), then you can set your own quality standards. And if you're able to make a game which is both fun and of reasonable quality in a reasonable amount of time, then you can make money from making a lot of lower quality games.
An example of this is Apple's Appstore. It's flooded with crappy apps as well as good ones. A lot of the time, the good apps can be overshadowed by so many bad apps that it doesn't really seem worth it. However, due to the shear volume of users of iOS products, you are basically guaranteed to get sales, even if it's just a trickle. And so, by coincidence, I am currently conducting an experiment of your very question. I actually released a game of 'acceptable' quality yesterday and got 8 sales overnight - which is absolutely nothing, but the game only took me 3 days to make, as opposed to my casual game on BFG which took 6 months and was of very high quality. However that game is there forever (until Apple closes the AppStore), and it will hopefully continue to have money trickle in. So if I made 10 of these, it would have taken me a month... and may be getting me 100 sales a day from all of them combined.
If you're selling games to flash portals (through FlashGameLicense for example) and getting a flat return, then you're not going to see this residual income unless you also have additional licenses in place. Here is a good example of residual income with a Flash Game: http://www.andymoore.ca/2010/03/steambirds-by-the-numbers/