Mordred did an okay job IIRC, but then it's impossible to tell how much help from others she had during that stint, and what were the exact conditions at the time working from a structure already placed by Artoria, and taking over for a short while is different from being the ruler for a long time.
And once Mordred's feelings and pride take over she pretty much drops most IQ points, at that point she still could function more maturely because she was more or less contented with her position. But the actual mark of maturity comes from dealing with adversity, almost anyone can act mature as long as things are going their way.
EDIT: To further clarify things, many DON'T act mature when things are going their way, but often that is a result of NOT wanting to be mature while on a good personal stride. Usually what those people need is a wakeup call, but the truly immature rarely learn when they receive one, for they will shift the blame on anyone else (Mordred's case) or resort to extreme self deprecation (I'm not worth anything!) as opposed to just assuming mistakes were made but you can learn from them and just do better next time (Artoria's case, as she thinks just erasing herself from history will magically fix everything when she probably can't even name a person she knew who would have done better than her, and ignoring that forces much wiser than her were the ones to deem her capable in the first place.)