In skits, songs, and other performances they epitomized the social skills fundamental to Sesame Street's mission--cooperation, understanding, tolerance and respect.You have to remember the time frame we're talking about here.
1963 - Gov. George C. Wallace stood in the doorway at the University of Alabama in an attempt to block two black students from enrolling.
1964 - It finally becomes illegal to post "no niggers need apply" in job announcements.
1965 - It becomes illegal to use poll taxes or tests to disqualify blacks from registering and voting.
1969 - Sesame Street
1988 - Congress overrides President Regan's veto of the Civil Rights Restoration Act making it ILLEGAL for private organizations that receive government funds to discriminate based upon colour (and so forth).
For that time, NOT being a small minded bigot WAS a pretty radical change.
And an "agenda" to show that "different" is NOT "evil" was very liberal.