I play ZSS but have 0 Fox experience. From what I can infer about the matchup, Fox has a strong enough escape game to avoid the dsmash quite well. The only things I can see ZSS punishing Fox with dsmash on are like...powershielded smashes or perfectly read/spaced ones on whiffed dairs. Other than that, it doesn't seem too likely for her to land one on him consistently.
The kill percents are kinda crazy as well, ZSS will die at like 80% to usmash and kill back at 110 with a harder move to hit with. As soon as Fox powershields a side B or dsmash from ZSS, it's a free kill move. Neither character is incredibly proficient at gimping the other, from what I can tell.
Also, once Fox has taken some damage, won't the dsmash chain become ineffective due to the move not being staled enough? Like, if Fox is at 40 and her dsmash is fresh, can she still chain him? If the lock is conditional to a low enough percent (obviously it is percent-conditional, but if it's something like 70 then this doesn't apply), then that's pretty rough.
I'd say it's even, if anyone wins it's ZSS.