Someone did a search with the query, "Should highly regarded be hyphenated?" The answer is no, and here's why:
When an adjective is modified by an adverb, as "regarded" is by "highly," you never hyphenate. With compound adjectives, however, you do hyphenate. An example would be "inner-motivated," as in, "She was an inner-motivated artist."
(In contrast, "highly motivated artist" would not be hyphenated because it's not a combination of two adjectives but of an adjective and adverb.)