People absolutely underestimate him, at first in my experience. This provides for interesting openings like this where you can actually surprise people with his unknown options, ie offstage spikes. Play has to get tighter as opponents begin to adjust to his unexpected options. What I will say is, I do not feel people continue to understimate my mac for long. His damage potential is quite nice, he can actually combo a down angled forward smash into running upsmash for 43% damage.
His ability to traverse lower platforms is in fact actually quite good. Battlefield is his best stage in most matchups in my opinion. His ability to lag cancel airs into smashes provides him with legitimate options, and opponents coming from below don't have a grab option, so one perfect shield into down-tilt Forward smash usually punishes pretty well. You can even angle the smash up to combo into an Up-b if they don't DI. The platforms from above guard from aerial attacks in this matchup and of course allow him to upsmash through. Also, naturally, the stage is set up for all those delicious stage spikes.
People neglect that he has does in fact have combos. His biggest strength is interrupting with lighter jabs and tilts to set up big-time damage. For this reason I assign c-stick to attack instead of smash.
What people are correct about is, that he can be combo candy and legitimately is very high risk vs reward. To do well you have to explore his options of return. Side-B early for mind games and damage (if you do this on the regular, you will die. Hard.), drop down up B a second time for dirty kills (Sometimes it jolts them up higher of only the first hit connects, killed a zelda at 70% with it) and surprise factor. One of my favorites that gets underestimated a lot is straight up standing up into ko punch, its quicker than most people expect. To be clear these are all high risk but you can't ignore the options because you absolutely need mixup to get back to your ground game.
In my opinion macs biggest actual weakness outside of recovery is his grab game. The reach is holding him back a lot, and its a shame because it combos (with bad di) into up B for significant damage and even kills around the 100% mark. There is a solid case though for "down tilt into running pivot-grab down-throw to up-b". His toughest matchup is actually Peach in my opinion, though he suffers somewhat against most "floaties" and naturally as does everyone else, "diddy" with his grab game.
I hope that helps! Let me know if so or you have any other questions!