Which Roman Catholic figure(s) has/have accused the Church of Mariolatry? I seem to remember hearing that an early R.C.C. leader had accused the church of falling into Mariolatry—the worship of Mary. Have any popes or other leaders of the Roman Catholic Church, especially in early church history, come forth with these accusations? 

*He may not have been an R.C.C. leader, but he was R.C. It seemed like it was a Pope, but I just can't remember.
Schismatics/heretics have said this. For example, the Collyridians sacrificed κολλυρίδας ("bread cakes") to her, and St. Epiphanius (d. 403) denounced this as mariolatry:


  Collyridians. Under this name [St.] Epiphanius (Haer. 79) assails certain women who had brought from Thrace into Arabia the practice of performing on certain days rites in honour of the Blessed Virgin, the chief being the offering of a cake (κολλυρίς), and the partaking of it by the worshippers. Epiphanius condemns their conduct because (a) women ought not to offer sacrifice, and (b) Mary is to be honoured, God only to be worshipped. The name Collyris (or kindred forms) is to be found in the LXX translation of Lev. vii. 12, viii. 26; II. Sam. vi. 19, xiii. 68; and the word passed thence into the Latin versions.


cf. ch. 6 article 1 of Mother of the Saviour and Our Interior Life by Fr. Réginald Garrigou-Lagrange, O.P., or the Catholic Encyclopedia article "Devotion to the Blessed Virgin Mary," § "The Age of the Fathers"