Early issues of Tom & Jerry depicted Jerry and Tuffy as being equal in age and maturity for the most part (at least to the point of Tuffy frequently seeking out love interests), it was less the ward or baby nephew he would become in later media. With both mice also frequently sharing colors for a not insignificant portion of the comics run I imagine the diaper was largely added to distinguish the two more often than not. Rarely is it ever addressed directly however (nor save for maybe on instance is self awareness made of it), but that being said in-universe what kept Tuffy in those cloth diapers all that time?

This is one of the later peices I did this morning, this time experimenting on not only simulating Ben-Day Dots but also the frequent occurrence of the color print heads being off aligned during portions of the printing process. Granted I just used one of the quick and dirty filters on CSP but I'm sure I could get a more dedicated filter or even a more accurate color mesh in the future.

Tom & Jerry are property of... MGM still? I honestly have no idea at this point