Bowser can't frame trap people in the air properly, and his edgeguarding is very limited. His best reward comes from edgetrapping or generically catching landings with SideB, which is less effective depending on the matchup.
Bowser only kills well if you're a character with very limited options to avoid his super specific situations where he can hit you. Link actually can get super massive reward in multiple situations, while Bowser has like...2 situations where he gets good reward that are extremely obvious.
Also uh, he definitely has basically the worst landing options with his landing lag and inability to really change his horizontal momentum meaningfully. At least Ganon Down-B is good at striking blindspots when he desperately needs to challenge juggles, but Bowser has nothing like that which can't be easily outspaced in most situations.
What are these super specific situations where Bowser can hit you? He has a dashgrab that can outspace hitboxes because of it's absurd range. Pivot grab can do the same and is very safe/ He has a long range unreactable dash attack, and decently fast aerials to catch jumps with.
Bowser can get massive reward in any situation except for edgeguarding. You kill people at 80% out of a long-ranged frame 7 jab if you have any rage at all, which you will if we're talking about top level play. He's like the best character in the game at getting kills from disadvantage because of how many options dair covers, and ff klaw onto someone trying to shield punish his landing.
On the ground he has frame 11 downb, which can combo from frame 7 jab, which in turn is quite safe if perfect pivoted. He has sideb, which kills early on platform stages, and usmash which isn't quick but is safe on aerial opponents if spaced correctly. If he has rage then all his tilts become kill moves.
In the air you have frame 9 bair, uair for catching double jumps, dair if they get too close, downb if they try pivot grab your landing, and sideb if they try shield it. With rage frame 11 fair also becomes a reliable kill option.
Also his best reward isn't punishing landings with sideb. If you're going to commit to a read like that, you might as well downb, which is only 3 frames slower, but does 24% and kills a lot earlier.
And no, Bowser does not have the worst landing options. His landing options are objectively better than Ganondorf's and DKs. Most characters can just shield against Ganondorf and even if they don't get a punish off it, at worst he'll reset to his terrible neutral.
There is also no way his landing options are worse than DK's. DK is pretty much always forced to face away from his opponent so he at least has the threat of spacing a bair, otherwise it's a completely free punish. At lower percents you don't get this luxury so you take a ton of damage. He's also way worse at the ledge. The only disadvantaged state where DK is better off is recovering.
There is no one option that is safe against Bowser because dair will beat anything that isn't like a Link upair, and shielding can be sideb'd, pressured with a firebreath landing, or even be broken with late downb. Yes, downb is an effective landing mix up in some scenarios, if you anticipate someone trying to pivot grab your landing or just dash-shield.
That's why Bowser is genuinely threatening this game, and is a solid mid-tier. Whether it be neutral or even Bowser's disadvantage, there is no one option that is always safe against him, and he has the ability to kill you with unreactable options even if you shield.