The quantity of new Pokemon follows the previous trend where even numbered generations have fewer new Pokemon but more evolutions for old Pokemon, except this time round they're called Mega evolutions, and functions more like formes than evolutions.
As for quantity and quality, I don't agree that having more Pokemon means that the overall quality must suffer. So yes, Black and White gave us 156 new Pokemon and you might dislike a couple of them. But that doesn't mean that the design on average becomes worse off, since each Pokemon concept is supposed to be viewed individually. Or at least that's how I look at them.
On the whole, there's easily more Pokemon that I like from BW compared to XY purely due to the sheer number to choose from. I don't even like many of XY's designs just because there's fewer of them.
Generally, I would think that most people prefer more Pokemon, if each and every single one of them fits their idea of 'good design'. Of course, that's never going to happen, so some people on hindsight wished that there were fewer Pokemon, or that those they didn't like never existed to begin with.
And honestly, I think some of you are confusing bad designs with simple designs. Grimer, Muk and Ditto are simple designs based off a very simple concept: slime. It's a cliche idea, but it works. As far as game characters are concerned, they look pretty standard to me, and any further additions to make them more 'complex' and 'creative' would have been uncalled for. Voltorb and Electrode are based on the simple motif of Pokeballs. Was it a bad idea to base a Pokemon off that? Maybe. But it still doesn't mean that simple = bad.