''More or less'' is a degree adverb (one that downtones the effect of the modified word), and it falls under the same class as adverbs ''somewhat,'' ''quite,'' ''almost,'' ''nearly,'' etc. I presume you already know that those words said to be modified in English constructions by adverbs are always adjectives, verbs and adverbs. These two sentences may perfectly represent constructions that take up the adverb under discussion to modify an adjective and a verb: ''Many people have an opinion that President Kikwete is more or less incompetent in accomplishing his presidential tasks'' and ''During my last month's visit to South Africa, I more or less developed a partiality for Kwaito music''