I'm not happy with the developed relationship or beef between Lusamine and her children either DatsRight (but I do like her character too.) Despite not playing the games, Lusamine had more reason for her actions while in the anime, it's been chalked down and simplified as Lusamine being a workabholic mother who made one rash decision (evolving Clefairy) without taking note of what her daughter would think. Sure they had good memories just as anyone else would with someone but with Gladion being away for a long time, it makes me wonder why (in the anime). I also think it makes Lillie look immature for holding a grudge against her mother over a one time event or treating her like a child when their problems and backstory has barely been explored. You would think with Gladion running off and Lillie being rude to her mother that they may have had more issues then what was said but I don't know what angle they're trying to convey in their relationship.
I think you're oversimplifying Lillie's relationship with her mother. True, it's not the same as it was in the games, but anime Lillie's issue with her mother is not just about her evolving Clefairy into Clefable, and it's not just about a petty grudge. Anime Lusamine is overbearing and condescending; she treats her daughter like a small child, and even calls her a baby. On top of that, she is not only a workaholic, but her workaholicism causes her to neglect her children. (On more than occasion, Lillie complained about her mother never spending any time at home.) Complaining about your mother being a neglectful, overbearing, and condescending parent isn't immature.
Whether Gladion had the same issues with Lusamine that Lillie did was never really explored in the anime. However, he
did call Lusamine out on neglecting her children; specifically, he made a point of calling her out on the fact that she never tried to learn why Lillie suddenly grew afraid of touching Pokémon. He also called her out on wanting to meet an Ultra Beast, even though an Ultra Beast caused Lillie's fear of touching Pokémon (remember, in Gladion's mind, Nihilego was the cause of Lillie's fear).
I do agree that Lillie and Gladion's relationship with Lusamine isn't quite as powerful in the anime as it was in the games, since game Lusamine was a far crazier character whose obsession with Ultra Beasts contributed to her neglecting her children. However, many of the key elements of Lusamine's relationship with her children—namely, her condescension and her neglectful behavior—
were carried over from the games to the anime.