I don’t know how well Mary plays the harp beyond a peculiarly becoming taste and expression but I think it is clear she enjoys performing and she knows how to do it to affect. She tells Fanny and Edmund:

“I shall be most happy to play to you both,” said Miss Crawford; “at least as long as you can like to listen: probably much longer, for I dearly love music myself, and where the natural taste is equal the player must always be best off, for she is gratified in more ways than one.” (7)

Mary is a picture to look at too. She is so perfectly framed with her elegant harp against the landscape beyond the window that it is clear it was consciously arranged to display her equally elegant person during performances:

A young woman, pretty, lively, with a harp as elegant as herself, and both placed near a window, cut down to the ground, and opening on a little lawn, surrounded by shrubs in the rich foliage of summer, was enough to catch any man’s heart. (7)

That “there was something clever to be said at the close of every air” (7) could be just as contrived as the theater Mary has created for herself. I think performance is truly the correct description to describe Mary’s playing. Mary likes to have Edmund near her:

Edmund was at the Parsonage every day, to be indulged with his favourite instrument: one morning secured an invitation for the next; for the lady could not be unwilling to have a listener, and every thing was soon in a fair train. (7)

Inviting Edmund back to the parsonage every day to perform for him seems to be right in the neighborhood of encouragement. Is Mary trying, successfully, to draw Edmund in? If so is she just amusing herself because his elder brother is not present? Could Mary’s decision Tom would do very well for her add something to her lively concern for the location of her harp and spur her unreasonable, most impossible attempts to procure its transportation to MP. As soon as Mary knew the harp’s estimated arrival date she tried to entice Tom back to MP through his brother:

“Now, Mr. Bertram, if you write to your brother, I entreat you to tell him that my harp is come: he heard so much of my misery about it. And you may say, if you please, that I shall prepare my most plaintive airs against his return, in compassion to his feelings, as I know his horse will lose.” (6)

Mary may dearly love music and be as gratified in the act as in giving joy to others but there is a great deal of description about every facet of her performance and in JA so much description can indicate something more than the obvious is going on: