It’s not a stupid idea, but it’s not something that should be undertaken without the family’s permission. Granted, it’s likely her ashes are mixed with all sorts of other people’s ashes, but folks don’t typically know that (or think about it if they do), and it’s not intentional. The intentionality is what makes permission-getting important. Maybe they want to make a piece of jewelry out of her ashes; maybe they want to bury them in a family plot; maybe they want to keep them on their mantle and mix their own ashes in there when the time comes. Whatever they decide, they should also get the option of deciding whether they want your burned shaved-off hair as part of that memorial as well.