Oh, he definitely can be good. Any character can be good if the player is good. But compared to other characters, he isn't very good competitively.
Bear in mind that this is considering the very highest levels of play. Unless you want to win big in tournaments without putting in a ton of extra effort, it doesn't matter who you play.
Not very good. It's slow and doesn't do much damage or put the opponent in a position that Kirby can really follow up from, so it's not a good command grab, and its potential beyond that really depends on how good the opponent's neutral-b is and what kind of benefit it offers Kirby. Using it in neutral is very dangerous, you kinda have to bait your opponent into running right into your mouth for it to work.
I would be very interested, since I know that if you know how to use Mac, you can and will get rekt.
I wish Mac was a viable character. I love the character, I love his design, I want him to be viable...
But he isn't. And short of a dramatic overhaul to his design or the competitive ruleset, I don't think he can ever be viable.
This is a character that is as one-dimensional as as a terminal vector. He excels in one thing, and that one thing is ground-based combat. The boy is
insane in his element. If you try to coast to victory on the proviso that "it's just Mac, he can't threaten me", you will get bopped. Unless you have a very reliable and safe disjoint or projectile game, fighting Mac to a boxing match is, nine times out of ten, suicide. You have to really shut down his options for a head-on confrontation to be anything but a
blitzkrieg to your stock.
The thing is that, if you don't want to play his game, he doesn't have much to pressure you into doing so. Mac is terminally easy to cheese. If you just want to say "lol jk" and bait him into disadvantage, and if you have the tools to put him out and keep him out, there's not much he can do to defy you, short of playing extremely cleverly and making sure he stays in his zone. He's one of the few characters (the others probably being just Ganondorf and Doc) that can be killed due to unfavourable knockback trajectory: you don't need to hit him off the blastzone, because most of his stocks will probably be dropped by not having the momentum to make it back to the stage. If he's knocked off-stage, the entire dynamic of the game changes: now he can't keep up the momentum and rearrange your face with his squishy green fists, now he has to shift tacks to just survive. When he has momentum he's great, but the second he drops it, he risks dropping the stock or even the very game. That is a crippling weakness to have in a game where stage control is vital to survival.
It's been said before, but Mac is basically that guy in a traditional fighter that can empty your vitality gauge in about five good hits, but whose gauge is half the size of everybody else's. You have to play constantly on point and absolutely minimise damage so you don't get bodied. One wrong move and it can be very difficult to recover. Mac is very true to his own game, in that he's a spry wee fella that has to duck and dodge strikes from opponents twice his size and strength and get creative to beat them back, because while he's got the guts to get the job done, he can't withstand more than a few hits himself. I don't think that kind of gameplan is sustainable in
Smash, where safety is king and half the battle is being able to make it back to the stage in one piece.
Mac, in my opinion, doesn't really have much room to develop. By virtue of his extreme design, the best he can hope to do is refine what he's good at and minimise opportunities for his weaknesses to be exploited - yet as the greater meta develops and people become safer yet more daring and better off-stage techniques become increasingly commonplace, Mac is going to find it very difficult to keep up and remain relevant (if he's even relevant now, which I don't think he is). He's a great counterpick if you're comfortable with the stage you're on and his matchup with the other guy is doable, and in that sense, I think he's probably one of the strongest (yet most niche) counterpicks you can have: if you know Mac is optimised for the conditions of your next fight, he can be a terrifying opponent indeed.
But as a solo viable main? Unless your name is Sol, I don't see it happening. Even then, I don't think Sol's ever been to a major, or if he has, he definitely hasn't done well enough to make big waves - Mac is inherently held back in his potential by the very nature of his design, and that's difficult to sustain in a meta dominated by characters that have great matchups with him. None of this is new, but I just wanted to offer my own perspective on the matter. Mac will body you if you don't respect him, but I don't think there's very many characters that really do need to respect him in the meta.
You can will get rekt if your opponent doesn't know how to fight him. If they do and he gets beaten in neutral, it's painfully easy to keep him from resetting.