Also, when you're both on the ground, a fmash angled upward does more damage (1% usually, not significantly more, amounting to 15% damage as opposed to the usual 14%) and angled down does less (again, usually 1% less). Headshot//knee shot.
I discovered this simple trick when I first started using Mario, and it's been infinitely useful. I've learned to angle stutter stepped fsmashes and turnaround fmashes, bypass shields and sweetspot every time (exaggeration). This is possible because upward or downward angled fsmashes go shorter distances because of their angles, and this knowledge combined with turned around fsmashes gives Mario quite the range to sweetspot with. Practise makes perfect.