Kind of both. On the one hand, she is definitely making an assumption, and it isn't the first one she's made that will turn out to be less than totally correct. She's stumbling in the dark and trying to figure things out from first principles, so mistakes are going to happen. On the other hand, her fixing spell DID work poorly on the plastic glasses frames, and there needs to be a reason for that. Also keep in mind that if the Wizarding World, as far as we can tell, have completely eschewed plastic, there might be a behind-the-scenes reason for it besides "plastic wouldn't fit the romanticized medieval aesthetic JKR was going for".