This is an older or more classical phrasing, so it's not really used outside of books anymore, except in one case (that I can think of.) That would be "ever so slightly." In that case (as in this case) it's just an intensifier to be used in order to emphasize the quality of the adjective. So, in this case, "ever so good" conveys the meaning that she is not just good, but very, very good (with good not meaning "skilled" and probably being closer to "well-behaved" in some way. Something that is "ever so slightly out of center" might be a few millimeters off of center, but is very close.

Hi, this is an old fashioned British expression meaning that the female is either very well behaved [ young girl] or if it's a woman that she is very clever or extremely talented at her work.

to put it simply, 'ever so' means 'very'.

Nick, "Ever so" has the meaning "vastly or immensely"; "to a great degree" She was ever so good. Thank you ever so much. Be it ever so humble, there's no place like home "Never so" has the meaning " to an unlimited degree or amount". Learning English was never so easy as it is today. Success is never so interesting as failure. I'm never so lonely as when I'm with you.

A synonym for "ever" can be "always" or "greatly." Basically, "ever" in your example would mean she is either "always so good" or "good to a great degree." Hope this helps! :)