Hmmm....Ashton is #109 in the SSA list for boys and not in the top 1000 for girls. I definitely think it's a boy's name.
Yes, definitely boy. Ashton has ranked somewhere in the top 1,000 boy names every year since the SSA list started back in the 1880s. Ashton for girls had a brief stint in the top 1,000 from 1986 - 2005. (yes, I say brief because that is nothing compared to how long it has ranked as male name)