If Mabel remarried , can Willem give her a title as countess. I'm sure on remarraige she would still be considered as a SIL/ Aunty by the very loving Dutch royal house.
Why would he?
According to the new (1994) Law on this subject, people can only be made noble by Royal Decree if they are members of the Royal House (ie in the line of succession and family of the Monarch until the second degree) or former members, but within 3 months after losing their membership of the Royal House. (basically to be read as: The Bee don't like new counts, The Bee won't make new counts)
Friso lost his place in the Royal House when he married (as did the others when they didn't ask for parliamentary consent) and he nor his wife were made anything other than prins van Oranje Nassau (see below), The Bee's sisters lost it when she abdicated, Maurits (and his kids) lost it when The Bee abdicated.
Margriet however is still a member (even if she;s related in the third degree to the current monarch) because she was part of it, and an adult, when the current Law on the Royal House came into effect (2002).
When you lose membership of the Royal House you lose the titles Prins(es) der Nederlanden and Prins(es) van Oranje Nassau, although you can keep the latter as a titre de courteoisie (without the perks) by personal decision of the monarch.
So even if he wanted to, he'd have to change the law - ain't gonna happen. He'd probably let her use the prinses thingy is she really wanted it.