Of the 4 categories you list, Mercy would rate thusly:
Offence: 85%: she's an aggressive girl who never shies from an attack. Quick thinker, too.
Defence: Here, not so swift. Mercy tends to get into sticky situations rambunctiously with no clear idea how to escape them. I'd say 20%.
Speed: Like a cheeta, her speed comes in spurts. Rapid at the start, but she tires soon. A 35% overall balance sounds about right.
Stamina: Mercy has a well toned body due to a childhood full of solo adventuring (hiking, climbing over fences, etc.) but no disciplined team training, since she's spent the whole of her life avoiding work situations. I think this latter consideration would hurt her for sports: let's say 35% there too.
All in all, probably not the strongest soccer player. The popular American cooperative sports of basketball and football (the US, not European, brand) likely wouldn't suit her either. She might excel at baseball, though, which has more of a dueling aspect (batter against pitcher) to it.