You are certainly right about the idea that slavery shows the hypocrisy of the Declaration of Independence. After all, the man who was the main author of the Declaration, Thomas Jefferson, was a major slave owner who never freed his slaves. A person who could write that "all men are created equal" while enslaving human beings is, by our standards, hypocritical.
I would also encourage you to think about the status of women in American society at that time. While women were not enslaved, they were deprived of many of the rights that women take for granted today. This went beyond just lacking the right to vote. Women were often not allowed to own property or to enter into contracts or to do many other things that adults are supposed to be able to do.
So, the men who wrote and signed the Declaration were talking about equality for all, but they were (you can argue) being hypocritical. They were willing to demand equality for themselves while denying it to others.