Sharing personal opinions or biases about a pet in a caption can, and is, harming their chances of adoption. It's important to remain objective and provide factual information that respects a potential adopters ability to make informed decision about a pet for themselves.
Even attempting to playfully overshare your opinion by saying, "I can't understand why this pet hasn't been adopted" you are actually low-key discrediting your organization. If you - a professional in pet welfare who is supposed to have a vast understanding of homeless pets - don't know why a pet has not been adopted, do you really think the reader is going to feel confident and inspired to adopt them? Or feel confident in your organization's ability to find homes for pets that need them most?
You might think you are saying "this pet is perfect, no flaws, potty trained, amazing on a leash and will make a great family member to any household"... BUT what your actual message projects to readers is "we don't know what is wrong with this pet since they haven't been adopted yet".