Calling my LO a Slang Name

I don't know if I'm overreacting or being rightfully pissed off. My husband's cousin started calling my 17 month old "Jughead" at first I'm like whatever. She said it's a series she's watching and it means "silly head" (translated)

Then, instead of greeting her she has started to use it as a nickname. She mentions today she wants to eventually get her a t-shirt with that name on. I couldn't take it anymore and said, to please not do that. I would prefer she gets her real and full name instead. That I don't like her to be called that. She insisted she finds it cute and fun.

Out of curiosity I goggled what it means and its actual definition by slang is "stupid person" ...mule!

I'm furious. My husband said to clear it up with her what it really means so she can stop. Why can't some people respect something when you tell them to stop though?