Why is Waluigi used in all of the WRs (at least for 200cc), does he give some sort of extra stat boost or something

Waluigi is rarely used by world record holders so I don't know where you're getting that from.

Metal characters are used for their speed, Rosa/King Boo/Link weight has good MT stats.

in most of the top 200cc run videos i see they are using waluigi/biddy buggy/leaf tires, you can look at the videos for things like 32 and 48 track runs, I'm mainly just wondering if specific characters grant a greater boost in some categories like it has in previous games like MKwii, where a certain character might give a greater speed bonus than another of the same weight class

In Mario Kart 8, characters are placed in several groups, and the characters in each group have their own set of stats. Sorted from highest base Speed to lowest base Speed, the character groups in Deluxe are as follows:

• Bowser, Morton, Mii (Heavy)

• Wario, Dry Bowser

• DK, Waluigi, Roy

• Metal/Gold Mario, Pink Gold Peach

• Link, King Boo, Rosalina

• Luigi, Iggy

• Mario, Ludwig, Mii (Medium)

• Tanooki Mario, Inkling Boy, Villager (M)

• Yoshi, Peach, Daisy

• Cat Peach, Inkling Girl, Villager (F)

• Toad, Shy Guy, Larry

• Koopa Troopa, Bowser Jr., Lakitu

• Toadette, Wendy, Isabelle

• Baby Mario, Baby Luigi, Dry Bones, Mii (Light)

• Baby Rosalina, Lemmy

• Baby Peach, Baby Daisy

Any characters that are listed in the same line share the same base stats with each other. If there's a gap/line break, that means the group below has 0.25 less speed than the group directly above it.

If you want to more the stats of each character group in more detail, you can either use this website to select and compare vehicle builds, or check out this stat sheet. The numbers here follow the 0 to 6 metric used in-game!

With that said, Waluigi shares stats with DK and Roy. This character group is often used in 200cc for having a good balance of Speed, Handling and Mini-Turbo power (MT is an hidden stat, just follow the Acceleration stat if you're confused!) when using builds that optimize mini-turbos like Biddybuggy + Rollers. It's one of the more common used setups for 200cc out there.