at the height of one's popularity; at the peak of one's popularity
Describes a person or thing at the very top of public favor. Often used in the pattern 人気絶頂の〜 or 人気絶頂にある.
He is now a singer at the height of his popularity.
That band was at the peak of its popularity in the 90s.
人気 is general 'popularity', while 人気絶頂 specifically means the very peak of that popularity.
絶頂 means 'peak' or 'climax' and can be used in other contexts (e.g., 絶頂期), but 人気絶頂 is a fixed compound for popularity peak.
Compound of 人気 (popularity) and 絶頂 (peak, summit). The exact historical derivation is uncertain, but the meaning is transparent from its components.