Both are 5-B, but Saitama doesn't start serious. It's explicitly a weakness on his profile.
Weaknesses: He doesn't take anything seriously initially, and has a tendency to hold back to possibly prolong a fight. Saitama has no true martial arts training, as he simply just fights, and is possibly unable to breathe in space.
So what makes us sure he'll go all out against a little girl before he's sure she can give him a good fight?
Although, likewise, even if Yukari could Matter Manipulate Saitama offensively... Reading, I don't see much reason she could, nor would; Don't Precures usually fight much more directly?
What can she do? She can summon her Crystal Animal. Does that offer any useful options?
She at the very least, can use Energy Projection to attack at range.
Saitama has Enhanced Vision so he can definitely see her attacks coming if they're within his field of vision, & with speed equalized, likely react to them, although he may not dodge until he's confident she can give good him a fight.
One question is: Are his shockwaves 5-B? They're a product of his attacks, but not listed in his powers & abilities. Yet several of his techniques are listed as making them.
His punches, Serious Punches, especially are capable of creating shockwaves, which would give him some range to fight back against Yukari's energy projection.
More relevantly are some of his techniques.
Serious Consecutive Side Hops lets him create afterimages (Does that work in Speed Equalized? If it does, it makes him harder to hit) & a wall of shockwaves as he advances.
Serious Table Flip will allow him to throw the ground into the air, creating a huge range of blind spots for him to exploit, or to literally throw the ground below Yukari's feet up, unsteadying her.
Saitama may not be incredibly refined in his fighting, nor academically intelligent & he may hold back a lot, but to say he lacks powers or skills besides punching is... well, I'd think it's a little inaccurate, "One Punch Man" or not.
That said, I'm waiting for more debate before I vote one way or another.