Having dealt with photographers for many years I would say that it is, but does not have to be. Obviously, there are more Spring and Summer marriages, school pics during school year, etc. Many photographers put their businesses in too narrow a niche and by virtue of that limit themselves to other opportunities. If a photographer is to build a name for themselves it will typically start in one city. They have to become known to a level that someone will say, "This family photo was shot by Bill Photo." But, there are many other areas to be explored: If a photographer is digitally savvy, the real estate market loves a person who understands what parallax is and how to best display a home for sale. They pay very well (Maybe a low of $150 for the lower end real estate up to $1,000's for high end homes). Be willing to set up drop boxes, etc. and teach agents and mls' how to retrieve photos and be prepared to assist them early in the process. Seasoned realtors are good at this, but new ones need help. You never know which new one will become a top producer.
Industrial/Commercial photography is another area where they can make money and reduce seasonality. Make it easy on them to get what they want and then have it available when they need a copy, etc. (Get away from the "I own the copyright and a copy is $500 BS - charge a fee, but make the money on the schedule time, difficulty of shoot and setting, etc. - no one likes to feel nickel and dimed).
Like any other business, you must be telling people you are there and constantly working the sales pipeline. If you are an "artist" and cannot be bothered with the mundane....good luck.
Best