Job Market Cover Letter - Mention undergrad experiences? I am on the academic job market in America in the social sciences. I am applying to R1/R2s, with a few liberal arts colleges sprinkled in. 

I've heard from a few people that mentioning undergrad experiences are a big no-no in both the CV and application cover letter. Specifically, I've heard that mentioning undergrad experiences makes it look like you have not done enough in grad school and need to pad your resume/cover letter AND that search committees don't actually care about what happened to you in undergrad. 

My undergrad degree was from a top school in my field (higher ranked than my graduate school) and I wanted to briefly mention what inspired me to enter this field. Is this a waste of space? Or worse, will I look amateurish?
Don't do this. It will appear as though you're padding, as you said, or it will appear as though you're defensive about where you went to grad school. (Also, your undergrad institution is higher ranked in what sense? While e.g. Yale is a very highly ranked school, many state schools have a better reputation for particular subjects at the postgraduate level.) The search committee can see where you attended undergraduate by looking at your CV.

How does what inspired you to enter the field relate to your ability to excel in the jobs you are applying for? Why is this information in your cover letter?

The only reason to mention your undergraduate experience in your cover letter is to address possible issues of "fit" or institutional culture. For example, if you have a PhD from an R1 but are applying to a SLAC, it can be advantageous to point out that your undergraduate institution was a similar SLAC and you therefore have an understanding of the institutional norms and mission of such schools. (Be sure that your school's really are equivalent though before you make such a comparison!) In other cases, I don't see much reason to mention your undergraduate institution in a cover letter.