I believe if a player wants to be truly successful in tourney play they have to pick up a higher tier character. This is true the majority of the time and people maining low tier who want to win should probably pay attention to whats in the first post.
While it is true a majority of the time, I cannot condone that first sentence.
Competitive fighting game metagames tend to be one in the same. Unless the character is ridiculously, ridiculously underpowered (for example, every character but himself chaingrabs him to death; dies consistently at 40%; etc.), they're usable and competitive, I say.
Forgive me for talking in theory, which obviously doesn't play out nearly as often as it "should", but let's look at a couple of tools that Bowser has going for him. (I'd go into detail with other characters, but this is a thread about Bowser in the Bowser forums.)
Koopa Klaw allows Bowser to be the only character to grab out of the air. The back throw is one of the most powerful throws in the game, and the forward throw sets up to a fair degree. It allows for everything you could realistically ask for in a normal throw (except for shaking out of the throw being more difficult at higher percentages), but you can do it anywhere. Think about how often your opponents on a higher ground (or even a lower ground!) attempt to shield the aerial they "know" is coming. Wherever the pace is at that time, it shifts more into Bowser's favor. If a Bowser player can condition himself to "know" when the opponent will shield, they could use it as a game-altering move that would change up expectations considerably.
Whirling Fortress is a "jack-of-all-trades", really; it prevents Bowser from being punished (at the right times, obviously), it racks up damage well, it KO's, it allows Bowser to stay mobile while attacking, it disrupts well, it allows for safer (and sometimes faster) edgeguarding in Fortresshogging, it eats through shields (in the air), and it is generally safe. While it's not foolproof (if you read this with another character's name there, you'd probably think it was!), it's obviously one of the best moves in the game, and arguably the best move in the game. It doesn't take a lot of thought to see why. Those who are not experts with Bowser tend to use the move very liberally, because regardless of whether or not it's foolproof, it's still difficult to counter most of the time. In the right hands, it becomes something for any opponent to fear.
I personally see the concept of "reading the opponent's mind" to be the most important skill in all fighting games that I would consider competitive. Realistically, I'm just talking about the Koopa Klaw here. The more than we have people who know players' psychology well enough to consistently Klaw through opponents' shields, the easier it would be for Bowser to be seen as competitive-viable to the general masses.