Due to their shield pressure, and spacial coverage; But also their very telegraphed jumps and approaches, this is one of the prime matchups you want to dash dance. A LOT.
Or in general just use your mobility.
Do not make a habit of approaching yourself, or charging FLUDD.
At least that's my running theory. I personally never liked this matchup, especially since Mario's style of edgeguarding isn't super effective vs them. (FLUDD is useless vs Tatsu, just gives them added height. Focus beats backair. They're hard to 2-frame due to the fat ledge-snap hitbox)
I do like spaced, grounded fireballs since it's actually very effective at 2-framing/harassing them. As they also lack the pure air mobility to avoid them easily, and their ledge snap is very hurtboxy. Plus if they go high, we're better equipped to punish with dair/sjp/cape, or even FLUDD if they try to focus through.
It mostly only maintains advantage and racks on hella percent, but I think it's optimal. Losing stage control vs these guys trying to edgeguard them, or even just getting reversal'd by focus/shoryu is just not worth the rare edgeguard.
This also pairs well with the 'dash dance' neutral since you're gonna be getting a lot of dash attacks, and the fireball harassing is pretty much the only consistent reward off that.