Revealing mistake: When Ernest is climbing up the chimney to get the mouse, the chimney shakes/wobbles slightly, showing us that it's made of rubber.
Revealing mistake: When the mouse chews the rope to make the little elevator fall with Catzilla in it, in the shot where the elevator is going to fall you can see that the cat has been replaced by a fake cat, before the elevator drops.
Answer: There's not really an answer, because the film has sort-of a nebulous timeframe given its cartoon-like atmosphere. Ex. Everything is old fashioned in design, including architecture and clothing, but current technology is also present. It's not really meant to take place in the "real world" or a specific timeframe, so much as be a silly throwback to old comedies and especially old Laurel and Hardy films. So, the best answer to your question would be that it just sort of exists in its "own" time, so to speak - an amalgam of different time periods all put together for maximum comedy impact.
TedStixon
Adding to this, the Wikipedia page on this movie states that... "The film is set in a humorously indeterminate 20th-century time period, with styles ranging from the 1940s to the 1990s."
Quantom X ★
"Mouse Hunt" was not modeled on 1930s Laurel and Hardy films so much as it was a direct retread of 1940s "Herman and Catnip" and "Tom and Jerry" cartoons, which spawned many knock-offs over the decades, from "Coyote and Roadrunner" cartoons to live-action comedies such as "Home Alone" and others.
Charles Austin Miller