It's probably less of a problem these days, where you can buy cards online - and many people consider them old-fashioned and don't bother now anyway - but traditionally, it's rough with getting birthday cards for a December birthday, with far less choice. Shops don't bother stocking cards that 1/12th of people might want when they can clear them out in favour of Christmas cards that nearly everybody will be buying. It's similar, but to a lesser extent, around Mothers' and Fathers' and Valentine's Day.
As has been said, it can also vary by how old you are. If you have an August birthday, for example (if you live in England or Wales at least), then you always get your birthday off school - but kids often like to celebrate their birthdays at school with their friends, and people are frequently on holiday and not able to come to parties. Then again, when you're an adult, you can often combine your birthday with a holiday - especially if you become a teacher; but if you want your birthday off work, you're also competing with your colleagues for the day, which you probably wouldn't be in April.
Does anybody 'do the maths' and specifically avoid TTC in, say, March, if it would lead to a birthday around Christmas? We were wise/sad/nerdy enough to do so(!) - but then, depending on your circumstances, you don't always have the luxury of choice to be able to waste a potential month.
FWIW, I personally think pretty much any time between March-July are ideal, but ultimately, you're born when you're born!