50:50
imo.
Kirby's got a low crouch, but Fox's tilt's can all hit Kirby fine (angle f-tilt down) and are all very quick follow-up starters. Kirby's advantage in this matchup is that he, unlike other characters, doesn't mind being right above Fox with all his multiple jumps and options to come back down safely. Both characters can combo each other fine, Kirby probably does more damage but that's the Fox life.
What you said about Fox being forced to approach Kirby can be true the other way around as well: Fox doesn't have to approach Kirby either. Both characters lack a projectile to pressure each other at range, so unless you've copied lasers, one of them is going to have to make a move eventually. Fox's movement in the ground and falling speed are also much better than Kirby's, he's got more angles to work with than Kirby onstage.
Kirby does win offstage for sure. Fox's linear recovery is easily intercepted, it's very hard to make it back safely at this point.
Each others U-smashes, b-airs, and u-airs KO each other. I wouldn't say Kirby has more KO power, both are equal in that regard.
60:40
feels more accurate.
Fox beats Peach. She's fairly easy to juggle as she lacks a good move to protect herself while coming back down. Easy to keep her up with U-tilt and U-air.
Her range vs his speed, Fox's speed wins as long as he doesn't get cornered, which isn't hard to avoid vs a slow character like Peach. Her turnip is meh for approaches and control, it's been that way since Brawl when it started disappearing upon hitting shields.
Peach definitely wins when it comes to edge-guarding. Stitchies also make me salty.