In light of Leon absolutely crushing Cosmos...
Has Bowser normally been considered a bad MU for Inkling? Like even with some sub-optimal play on Cosmos' part that was just brutal. Leon didn't lose a stock for how long again? Sheesh.
Inkling vs Bowser isn't really a bad MU for Inkling and I believe most players think it swings positive for her. The problem is Inkling is generally denied one of their most potent neutral moves, which is rapid jab.
Cosmos straight up did not know the MU, he admitted that much and you could tell by multiple of his plays/decisions in the game that his net inexperience snowballed. For one, switching to Corrin - that was a pure panic move when he settled on his Smash 4 complacency; "swords are the easy answer" as we so often see. But more importantly was how he used his Inkling approach options. There were multiple times in set when Cosmos attempted to jab Bowser, but instantly got cold feet; backing off even after successfully connecting one jab. This is already when it was clear LeoN was in Cosmos' head, because he was too afraid to use what is one of her safest neutral options. Despite rapid jab being an extremely good move to lose in this matchup, the gentleman is indispensable for a matchup like this in order to keep Bowser honest. I can't recall a lot of DTilt usage (not sure myself if it low profiles anything in Bowser's kit, or if it can setup a read/combo situation given his frame).
There was also very little Splat Bomb usage; a tool that is a must in the MU (especially when you can't use jab to ink him) and keeps Bowser's neutral in check. Despite Bowser's great ground mobility, his air mobility isn't the greatest, so Inkling will generally want to use Splat Bombs to cover her mid-range coefficient while spacing with yee traditional BAir usage. Inkling's main goal in this MU is to get Bowser off-stage; that's where he struggles, and gets susceptible to edgeguards and frametraps.
Cosmos' biggest mistake was generally autopiloting off of the traditional assumptions that heavies are easy targets and as such going for way too many grabs and aggressive landings (and I guess counterpicking Town & City twice which - I know it's Cosmos' favorite stage but - is very much a double edged sword against a character like Bowser, not just because of survivability but because those platforms benefit him). I can't blame him for that because grabs are how you get Inkling's most damaging strings, and Bowser being a massive target means he's subject to a bunch of custom combos that are unique to him. But that aside, Inkling has a fairly modest grab range that's dwarfed by Bowser's. Because of that, misspacing or abusing grabs can ultimately be the factor that decides your entire stock.
Heavies like Bowser want to get into your head, they want you to regress into a "land aggressively/dodge liberally" mentality. We generally see this with players like Nairo, including when he goes Ganondorf. LeoN was very much in Cosmos' head, and when you don't know the MU, that's when you've already lost. Against Bowser, with Inkling, you need to play a little lame, since aggro is what he wants. Cosmos generally doesn't do that, or at least hasn't done that.
From what I hear, LeoN has a lot of Inkling experience in his locals, so I'm not too surprised if so that Cosmos' panic Inkling strats did not pan out.
EDIT: On a separate side note, despite that legendary beat down, this is the first national in a while that had some pretty good Inkling representation in Top 48. We obviously had Cosmos, but there was also Rags (predominantly an Inkling main but he also used both DK and Greninja this tournament) who went out at 33rd, and also sinnyboo242, who went out at 33rd going solo Inkling. There was also Wishes, who jumped back to his old flame for a short while after being down against Zack's Bayonetta - and frankly did better against Zack's Peach than his PT. Overall, that's pretty good representation in a 1600 man tournament, given that people often make a point of her quantity being lacking, as well as not a lot of top Inkling players being US-based.