I have read an argument from a Taoist point of view, that eating both locally and seasonally is a 'good' thing to do. The article went on to suggest that it is 'natural' to put on a little winter fat when winter vegetables, soups, and broths are available; and that the best time to diet was indeed the summer months when salads are in season. Now, Taoist beliefs aside, it does seem to me that eating fresh, local, and inseason whenever possible would be a good way eating - for best nutrition values, and for the environment.
I wouldn't take it too seriously though, I do like my bananas, walnuts, citrus fruit etc around the year - and we don't get too many of them growing in wintry English gardens.
As for the argument that it is 'natural' to put on a little winter fat, well, yes, I go along with that - but I am sure that depends on how seasonally you adjust your diet - you can these days buy out of season salads around the year. You can go against nature and design a winter diet low on saturated fats - lots of steamed veggies etc, and cut back on any fatty broths.
I would stand by the line that it is easier and more natural to lose weight during the summer months. Having said that - weightloss is about taking control of what you eat, and how you live.