If this is an operation on an IList<PointF>, then it should be an extension method on IList<PointF>.
Generally, Utils and Helper type classes should be avoided. More often than not, you will find that what you may think is a utility method, is actually a rather specific method that probably belongs in a class of its own (just like you say). However, there will be domain specific cases where Util-like classes (classes which group related useful methods) are valid entities.