How Did This Happen and What Does It Tell Us?
Choose your own, from our list of possibilities...
1) Garfield did not see the mouse and came to rest where he pleased. Insight: Garfield makes no effort to be aware of his surroundings, as long as his own needs are being met. Also, a fat joke in which Garfield's body mass blocks his line of vision and causes him to inadvertently harm others.
2) Garfield saw the mouse, specifically chose this spot that he could crush the mouse. This also means Garfield is lying to the second mouse about the brother's whereabouts. Insight: Garfield enjoys inflicting physical and mental anguish on others for its own sake, habitually abuses smaller creatures and lies for no larger profit, and is compelled to demonstrate dominance and ownership of the house.
3) or 1+2 Garfield chose his nap spot, saw that it was occupied by the mouse, sat on the mouse anyway. Insight: Garfield does not care what suffering he inflicts upon others, as long as his own needs are being met.
All these things are true about Garfield's character, regardless of our plot speculation.
Bonus Game: Try to reproduce the muffled voice of the squashed rodent. You will find that the only way these mangled pronunciations can be created is by stuffing your mouth with a wad of soft material.