Sugoi roughly means "amazing" in Japanese. The adverbial form is sugoku, which can be used as an intensifier with a meaning close to "very" or "awfully". But chaining sugokusugoi, while grammatically correct, can sound a bit odd to Japanese ears because of the redundancy. It's somewhat similar to an English speaker using odd adjectival-nouny or verby-verb word structures when they can't quite recall the right vocabulary to describe something.
Narita Top Road frequently falls back to sugokusugoi when, well, praising anything. It's practically her catchphrase. So there's a joke among the JP fanbase about her suffering from a vocabulary deficit, and other Umamusume characters using the same sugokusugoi construction sometimes get described as using "Top Road Vocabulary".
