The Garfield Movie answers the age-old question.
The negative reviews could be due to the non comic-accurate backstory that was given to the orange feline.
Whatever the reason, the film still provided a detailed answer for why the famous cat hates Mondays.

Sony Pictures
Who Doesnt Hate Mondays?
Toward the beginning of the film, Garfield gives the audience a mini overview of his life.
Garfield does not care for this particular comment that low-key throws shade at his weight.

The first day of the work-week also brings about bath day.
Garfield is seen complaining throughout the entire mildly aggressive washing and drying process.
Its a funny pet-centric explanation for why a spoiled house cat could possibly hate Mondays so much.

Sony Pictures
But it works due to the levity with which its executed.
September 18, 1978, was the first time Garfield expressed his dislike for Mondays.
Over the years, fans have shared various theories about why Garfield hates Mondays.