Goddess was converted in archaic times but still carried the meaning of giver of life. She then devolved in Mother from Goddess. When you see Mother of Thunder, it refers to creator of thunder or thunder itself. From Wikipedia: Perkuna-tete or Perkunatele in Baltic mythology the Thunder Goddess Mother, in Slavic mythology she referred to as Percunatele mother of Perun, which is probably derived from the Balts. Like many such goddesses absorbed into Christianity, she is, today, the Christian Madonna, Mary, one of whose epithets was Panna Maria Percunatele. She was originally derived from the Baltic Thunder Goddess.
In the book: About the Faith of Old Slovenes by Nikola Grzhteetić, he mentions Perunika but I cannot read Slovenia text. There is also Marija Gimbutas’s book the Gods and Goddesses of Old Europe: 7000 bce to 3500 bce.
Gudiri-mumi – Russia – Thunder Goddess
Rauni – Finland – Thunder Goddess
Percuna-tele – Poland – Mother of Thunder