sorta.. kinda.... yea.... It is not quite as simple as that.. but your in the ball park... Purity is not correlated to antagonistic activity.
Purity estimation by ESI-Mass spec reflects the relative amount of the target molecule with respect to total amount of molecules in the sample that are all ionized in the positive ion mode (when protonated by formic acid), excluding negative molecules. High purity means less other molecules present in the sample, low purity may or may not related to the efficacy of the molecule if other molecules are having no antagonist activity. There should be a concern from low purity sample where degradation products, or derivatives from active component are abundant and whose biological activities are unknown, which also indicates the quality in the process chemistry in manufacturing the product. So high purity is desired, low purity if other molecules contributed no biological activity, then is ok, otherwise, the low purity is a sign of not as good quality.