In the latest hotfixes, the Season 4 Fated Raid achievements have been changed to clarify that the Slime Cat mount will only be available if you complete the raids on Normal difficulty or higher.
A couple days ago, Blizzard clarified that the Slime Cat mount will not be available on LFR difficulty during Shadowlands Season 4. This came as a surprise to a portion of the community as the achievement made no mention of this requirement.