Is it just me, or is Mario a much harder human opponent than Link?
Many human Links don't have time to just stand and block your projectiles, and they definitely won't always have the timing to shield anything else you throw at them. I usually find that human Links will take a much more offensive approach than the Level 9 CPU because they don't have the same shielding skills, leaving themselves open to your arsenal.
Human Marios, on the other hand, are way bigger threats than Level 9 Mario. They won't just shoot fireballs at you like a dumbass until they realize it won't work, they'll flat out try to make sure you'll can't implement your lemons at all. They'll still go for that annoying Down Throw + U-Tilt combo, although you can escape out of it since you're god damn Mega Man. You can't Side Smash them while they're off the stage because of the cape, although that's something you really couldn't do on Level 9 Mario either. They also rack up damage at a way faster rate than you do. I remember dealing 80% to a friend while at 5% myself, only to be brought up to the same percentage as him because I didn't rush cancel effectively.