Is a home improvement contractor liable for sales agent representations regarding work to be performed and appearance of final product?

That's probably just wishful thinking on the contractor's part. The sales rep was their agent at the time he made those representations; the representations were (presumably) both reasonable and within the bounds of what you'd expect a contractor's agent to represent (e.g. he was not promising anti-gravity garage doors or that redoing your kitchen would result in 10,000 percent return on your investment, neither of which credible and which the contractor could disclaim); you were (again, presumably) reasonable in relying on those representations, which induced you to employ the contractor.

When a business's agent or employee makes reasonable representations as part of his job and which any reasonable outside person would believe are within the rep's authority to make,--and the outside person in fact relies on them--the business is bound by them. The business may choose to sue the rep to recover their costs, but that does not make them not liable to you.

Only if the representations were not reasonable or believable, or there was some good reason to think that the sales rep did not have the knowledge or authority to make them (check the paperwork you signed; any caveats or disclaimers in it?) could a business credible disclaim those representations.