And where/how are you trying to sell it--your own URL, a marketplace like Etsy/Amazon, or a POD site like Redbubble/TeePublic? (Don't post a link here.)
You need to think about your keywords/titles and what search would bring a potential customer to your listing. Customers are searching for a gift for Uncle Bob who may in fact be "one in a million," but is that what they are going to type in the search field? I doubt it. If they know Bob spends all his spare time golfing or has a Golden Retriever, then they are likely to use search terms related to such topics, like "funny golf shirt," or "Dog lover gift," or "Golden retriever gift."
As that example illustrates, it is harder to market generic humorous designs than ones that play to a specific niche. You can festoon that listing with generic terms like "birthday gift shirt" or whatever, but then you'll be competing against ten-thousand-billion others using the same overly-broad terms.
And that said, there are probably a million listings for Golden Retriever shirts and a billion for Golf shirts. Pickings ain't exactly slim for shoppers looking for novelty shirts, as with the advent of POD the barrier to entry to this industry is now effectively ZERO, so anyone can, and does, do it.
You have an advantage if you know and care a great deal about something--insider knowledge--and have the wit to turn that into humor that would be understood and appreciated by those with the same interest. There may not be millions of people into roadkill taxidermy, but those who are will greatly appreciate your insightful take on the hobby.
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