i don't watch any specifically mario-focused channels, but i figure since other people are shouting out tangenially related gaming channels, i'll put in a plug for one of my favourite youtubers,
Game Maker's Toolkit.
GMTK is a really great channel that makes video essays exploring various gameplay concepts and explaining some of the thought processes and design choices that come with them. They're always well thought out, and draw from real games to provide examples and explanations of what he's talking about. He's also got really good production quality and motion graphics which helps make it all easier to digest and understand. A few of the videos he's made even have some
footage of interviews he did with the game's developers for the videos, which is really cool.
In terms of Mario content, he has a few really great videos about level design, looking at games like SM3DW, SMM1&2, and DKCTF. The DKCTF one talks about how the game takes Mario's level design philosophy (he explains a basic version of this in the video, but if you want a fuller explanation then check out the SM3DW video) and builds on it to make levels that are much more complex and well-designed (my favourite video of his, and probably also one of my favourite videos about DKCTF as well, but i'm probably a bit biased

). There's also a good one about Rayman Legends where he discusses a different approach to designing levels and worlds by looking at the 4th (and best) world, 20,000 Lums Under The Sea. They're really great videos so I'll leave a link to both of them and a link to his playlist of other platformer-related videos, where he talks about other games like Mega Man 11, Celeste and more.
Apart from that, he's got all sorts of content on there. He's got a series called Boss Keys which looks at dungeon design in Zelda games for the first season, then design of the world maps in Metroid games (plus SotN and Hollow Knight), there's videos about designing for people with hearing, visual, motor and cognitive disabilities, and just a bunch of stuff that's standalone content. If you're interested, I'd recommend just going to the videos tab on his panel and scrolling down to see what interests you; his videos typically have enough info in their titles/thumbnails to be able to know what it's about at a glance.