>only creates abominations
Chara, darling I think you are forgetting about someone important

How can you ever say that legs like these could be an abomination

Neko is the japanese word for Cat.

An exclamation is commonly used to denote a specific adjective or universe said character is from. So Underswap!Sans would be Sans from the Underswap universe. Similarly, Determined!Undyne would be used to refer to Undyne in her sort of Rage mode.

Amalgamates are fusions of monsters and typically look like abominations

Neko frisk. Essentially a catgirl.

Frisk said that Alphys made them look like a cat.

Chara is saying that Alphys can only make abominations, therefor Frisk is an abomination in that form.