Grandma bike specifically refers to bikes with retro looks or the original designs that the retro bikes take their inspiration from. It doesn't refer to utility

Grandma bike specifically refers to bikes with retro looks or the original designs that the retro bikes take their inspiration from. It doesn't refer to utility bikes in general, at least I've never heard it used in that way. So it's not derisive at all, just referring to "how bikes used to look 50 years ago."