Yes, you are correct, it is an adjective in the superlative (because of the word "most" in front of it). Note that participles in the comparative or superlative usually take "more" or "most," even if they are short enough to take -er or -est. Example: "More fried," not "*frieder." The expression "deader than a doornail" is an exception (a set phrase that's been around for centuries).