Which MUs are worse for Small/Thin Brawler, and why plz?
That matchup list is pretty outdated, but i'll try to give you my MU experience now.
Tiny Mii Brawler's biggest weakness is his lack of range, if he can't grab you, he probably won't be able to win. None of his attacks have any sort of range and I'm sure you've noticed how small of a hitbox moves like upsmash have. That being said, you need a character who can.... well, not get grabbed while at the same time out range him and punish him for whiffed grabs.
These characters are Luigi, Jigglypuff, Pacman, and Peach, and these four characters have frustrated me more than any other character. Luigi's jab and aerials pretty much beat anything tiny mii brawler can do and his own d-throw combos can kill at higher percents unlike Brawler's, Angel Cortez quickly picked up the character and gave me a TON of trouble.
Next is Jiggs, Jiggs is a character who's playstyle is playing safe and punishing her opponents for their mistakes, in this case, punishing Mii Brawler's whiffed grabs, a good Jiggs player has beaten me in friendlies plenty of times due to this, however, you have to sort of play on point because Brawler can kill her super early, but if she lives past like 60% she's pretty much good and the matchup will be in her favor.
Pacman is annoying as well due to his keepaway game. Generally you want to use the on-fire hydrant and the meteor trampoline custom to keep Brawler away from you as long as possible, however, this strategy too requires you to be pretty darn efficient with Pacman, so probably not advised if you don't actually main him.
Finally we have Peach, god she's annoying to fight, similar to Jigglypuff her floating ability makes her so damn safe and she can punish every single whiffed grab. As a Brawler main you have to play on point and figure out when you can get grabs when you can. I fought a player named Ling-Ling who used peach and knocked me out of winners, I had to play incredibly spacey and lame to beat him in Grand-Finals and it was not easy.
With that said here are my MU ratios
Luigi: 4-6 Luigi's Favor
Jiggs: 55-45 Brawler's Favor
Pacman: 6-4 Brawler's favor
Peach: 4-6 Peach's Favor
Captain Falcon, ZSS, Villager, Yoshi are about 50:50
Diddy/Sheik/Pikachu/Ness/Sonic/Fox/Mario are all solidly in Brawler's favor
dunno about Rosalina, but if you can play as good as Dabuz, its in her favor.