"No! Bad dog!"
Translation:Ei! Tuhma koira!
To be grammatical, I believe that would need to be either Minulla on kissoja, jotka ovat melko tyhmiä, or Minulla on kissa, joka on melko tyhmä.
If kissa is nominative singular, then the relative pronoun joka would remain singular, and the following verb must be singular on.
If you have multiple cats as the subject of an existential/possessive sentence that starts with the location/possessor, I've read that they must be partitive plural, kissoja. This is mentioned at the end of https://uusikielemme.fi/finnish-grammar/syntax/sentence-types/existential-sentences-jossain-on-jotain.
I believe you've seen the partitive singular endings -a/ä, -ta/tä, and -tta/ttä. The partitive plural endings are -ja/jä, -ia/iä, and -ita/itä. The word kissa uses the partitive plural ending -ja, resulting in kissoja. In contrast, Tyhmä uses the partitive plural ending -iä, resulting in tyhmiä. There's more on the formation of the partitive plural on https://uusikielemme.fi/finnish-grammar/finnish-cases/grammatical-cases/the-partitive-plural-monikon-partitiivi#two.
Now, if the subject is partitive plural kissoja, then the relative pronoun needs to be nominative plural jotka, agreeing in number with its antecedent. Notice how plural jotka has the plural marker t after the root jo-, while singular joka does not.
If the relative pronoun is plural, the verb is indeed plural ovat. But since the subject and verb are plural, the predicative adjective needs to be partitive plural tyhmiä. This is mentioned in https://uusikielemme.fi/finnish-grammar/finnish-cases/grammatical-cases/the-partitive-plural-monikon-partitiivi#one.