Learn Swedish: Katten är väldigt sällskapssjuk idag.

It is a compound word made of sällskap (company) and sjuk (sick). Literally meaning company-sick, it perfectly captures the feeling of being lonely, craving attention, or, as translated here, being clingy.

Could I use mycket instead of väldigt in this sentence?

Yes, you can. Both mean very and are largely interchangeable before adjectives. However, väldigt (related to the word for enormous or mighty) often adds a slightly more expressive or dramatic flair than the neutral mycket, which suits an emotional word like sällskapssjuk.

Are there other common Swedish adjectives that end in -sjuk to describe a feeling rather than a physical illness?

Yes! A very common example is svartsjuk (jealous, literally black-sick). You will also see pratsjuk (talkative, literally talk-sick) for someone who desperately wants to chat. In these compounds, -sjuk indicates a strong urge or emotional state rather than a medical disease.