Translation:My cousin has the messiest kitchen.
So "rotete" doesn't have a superlative form proper? Are there a lot of adjectives like this -- where you're supposed to use "mest + adjective" as opposed to "adjective + st(e)"? I realize that this is (fairly) often the case in English, as well. I just find it rather surprising that this is also the case in other Germanic languages =0
In addition to the rule about an adjective requiring an -e on the end when combined with the definite form, my research into this word (along with practice within Duolingo) has led me to believe that rotere is the base form of the adjective*, and that there is no adjectival form 'rotet' for indefinite masculine or neuter nouns. (The word rotet does exist in Bokmål, but as the simple past or past participle form of the verb å rote 'to mess up, to clutter,' - Han har rotet på kjøkkenet - or as the definite form of the neuter noun rot, meaning 'root' or 'mess', 'disarray'.)
*This is of course subject to correction by a native or more advanced speaker, as always - if I am wrong, please tell me!