I hate to say mid but we're talking about a game where the low tier is almost entirely indistinguishable in the non-customs setting. The bigger issue is how all the low tiers have caught up to Bowser in a way that it's a challenge to pick out what our truly advantageous matchups are. Robin, Ike, Samus, Zelda, Marth, all those guys are way more well rounded now than on release. Not better than Bowser, but they're catching up to the point where they are better picks. All Bowser has gotten is Flying Slam control, A serviceable Utilt, and slightly more rewarding Dsmash and Nair. Fire Shot is exponentially better too, but a lot of us don't play with customs these days. And we certainly don't have advantage against the highs, because if we were even half with them, we'd be there in high tier. I feel like Bowser has a reasonable time against Diddy and Rosalina, but that's the most optimistic I can get on high tiers.
I will say Bowser is easy to pick up if you've got at least the basics of Smash down. He rewards smart timing and respect of the opponent's options. On my first three weeks with Bowser I placed higher than I ever had at my local tournament, and I just made heavy use of Jab and Bowser Bomb. I didn't know about Side B tricks, when to use Fire Breath, and how to stay grounded and control the stage. I had to quit competitive soon after, but Bowser's potential was clear, so I started labbing. Read up on our threads, ask questions, and fight people with Bowser to see how well he fits.