Yes, UGC is a ranking signal, according to Google’s Gary Illyes:
@jenstar it feeds into general quality. Say, there's good content, 5 points, great links from great pages, 2 points, thriving community, 1pt
— Gary "鯨理／경리" Illyes (@methode) April 27, 2016
Notice how Gary says that UGC feeds into general quality. As long as it’s moderated to remove profanity, hate speech, etc., and if it genuinely adds to the page quality, UGC (such as blog comments, product reviews, etc) can positively impact rankings. The moment UGC is uncontrolled, it can become a negative signal.