Are light periods in a woman a sign of infertility?

No not at all. Menses is not fertility. Light periods could be just the normal pattern for some females, related to the use of hormones e.G birth control pills, use of some types of iud... What ever the cause may be light periods do reflect or predict fertility at all.
Probably not. If you are having regular, but light periods, you are likely making an egg every month and are probably fertile.