I'd accept it and say that I appreciate it, but also mention that there may be other things that could be more practical (in a nice way of course). You can also let your MIL know your intentions, because sometimes all they see are just us not appreciating things!
My parents buy things for my son ALL THE TIME. If they were practical, fine. But they buy cheap toys that cannot last long, most either spoil within a few days or isn't even suitable for my son in the first place. I give them warnings here and there, but if it gets too frequent I'd just tell them off because all those toys end up getting damaged and thrown away within a week - not only waste money but waste resources and create trash.