There is one subject (It) and one verb (looms) ... so yes, it's a simple sentence.
Your analysis is correct! The sentence "It looms menacingly over the road from the roof of a farmhouse, a lying reptile with a seven-foot wingspan" is a simple sentence. A simple sentence consists of one independent clause, which is a complete thought that can stand alone as a sentence. In this case, the independent clause is "It looms menacingly over the road from the roof of a farmhouse." The additional descriptive phrase, "a lying reptile with a seven-foot wingspan," provides further details but does not alter the sentence's basic structure as a simple sentence.
