Name Calling

Seiko's has to be one of the more... interesting linguistic theories put forth in this thread. It's fun to ponder, but there's other, more plausible explanations why the host mother may have though "stupid" to be an acceptable jab. Manzai style comedy for example features this style of put down pretty extensively. It is also both older, and culturally more significant than hip hop in Japanese culture.

It's probably just a bad translation.

Limited English vocabulary, or misunderstanding of the meaning of 'stupid'.

Don't worry about it, unless she said it with a nasty tone and beat you with a stick at the same time. That might be wrong.

Is it beyond the realms of possibility she was calling the rice cooker appliance stupid rather than you?

And if not, it's certainly possible that stupid was a loose translation of daft, silly, or some other much softer chastisement.

It's often a good idea to look at intention rather than the words. e.g. students are apt to say to me "I want you to..." which at face value sounds like they're being rude. But it's just that they don't know how to express themselves properly. It's easy enough, as their teacher to tell them how to match their intention with their language.
In the case of living in a different culture, it's often wise to keep a an open heart and open mind and give people the benefit of the doubt. If she says something that troubles you, why not try and clarify it at the time. As others have said, if she otherwise seems quite reasonable, cut her some slack.