AI am making a cookie monster cake for my friends birthday I am going to make a chocolate cake and fill it with a recipe for cookie dough filling I found on this site. To decorate the top of tge cake I have a grass tip #233 which should hopefully give the right effect. For the top and sides of the cake should I just use a plain butter cream and color it blue?

yes that's what i've done for cm just make him look all raggedy

and for the buttercream yes just a powdered sugar buttercream because the overload of piping and the heat from the hands will wilt a lot of meringue icing fast -- but you could use that too but it'd be more ticky to do--keep your hand in a oven mitt to pipe will keep the heat off your icing to a certain extent --

AAlso if you are using american buttercream with shortening, don't let your crumb coat crust as your "fur" won't stick to the cake and will keep falling off, esp on the sides :( learned this the hard way!