Whether you are justified in being mad at your dad depends on various factors and the dynamics of your family.
It's natural to feel upset or frustrated if someone takes money from you without your consent, especially if it was for a purpose you didn't agree with or wasn't discussed beforehand. If your dad took the money without asking and didn't have a valid reason or explanation, you have a right to be upset.
However, it's also important to consider the context and your family's financial situation. If your dad took the money to buy expensive watches for your cousins as gifts, it might be because he wanted to show his love and appreciation for them, even if you don't personally agree with the decision.
Open communication is essential in situations like this. It would be helpful to talk to your dad calmly and express how you feel about the situation. Try to understand his perspective as well. Perhaps he had good intentions, but he may not have realized the impact it would have on you.
In the end, family relationships can be complex, and sometimes misunderstandings or differences in priorities can occur. Talking it out with your dad and finding a way to resolve the issue together may lead to a better understanding between both of you.