I have never heard the word "Monday" used in that context before (as a nickname or insult, racial or otherwise). I guess I should browse Urban Dictionary more often.
Given the current state of affairs, uncommon racial slurs are probably the nicest things being said about Crawford on the streets of Boston.
If it was meant as a racial insult, and from what I've read so far online about it's use in police circles, it was, then obviously it's no excuse. What's ridiculous is that it has happened before and it took this guy using the term on a famous person for action to be taken. Granted, I am usually a strong supporter of law enforcement (for obvious reasons), but I know that there tend to be less progressive views on minorities prevalent among departments in big cities. Prevalent in the sense that those who share the views draw themselves together, not in the sense that the whole department is racist.
Personally I believe that the number of racist pricks in the ranks of police is likely a representation of the population as a whole. The reason it seems more prevalent is that, much like incidents of criminal behavior and general assholism, you hear about it much more often due to their position.
Just remember, folks. For every story you hear about a police officer doing something stupid, illegal, or immoral, there are a hundred unreported stories from that same department about other officers clocking in, doing their job quietly and respectfully, and clocking out. In other words, no news is good news and bad news is site traffic (or newspaper sales, but honestly, who left without tufts of ear hair still buys the newspaper?).
To make a long opinion short (oops, too late), don't let the actions of idiot police officers make you distrustful or hateful of the police. They are there to enforce the laws and take the bad guys down, and while some of those laws may not necessarily be agreeable, because honestly, I should be able to urinate wherever the hell I want you commies, on the whole our society is better for the 95% of these men and women who go out there every day and do their jobs correctly and with skill.
This has been a public service announcement from your friendly neighborhood narc.