Do you have any sets to support your opinions?
i dont have any videos.
marth: the main thing with this MU is that ZSS just has a way better time getting conversions. she outpaces marth a good amount, has a low commitment projectile, and relevant disjoint to contend with his. marth getting a grab does get him some juggle opportunity, but it doesnt take him as far as other MUs because ZSS has burst movement with down b as an aid against juggles. when it comes to edge guarding, marth is honestly not great at edge guarding ZSS. dtilt or dair edge guards are completely negated. He has a terrible punish game on tethers. contrary to popular belief, its extremely difficult to time a dair and get a spike off. ZSSs reel in is approximately 10 frames, which is well below a normal reaction time. nothing he has gives a good punish onstage either during the forced hop (as opposed to punishes from characters like fox, roy, falcon, wolf, etc. that can link right into direct kill moves with almost %100 certainty). all in all, ZSS wins neutral more often with a higher mobility and a great projectile, punish game favors ZSS, and survivability favors ZSS as well. marths punish game gets a bit better on FD, but neutral still doesnt get too much easier for him. Smaller stages also help out marth since he can zone out well and has a better chance of landing an fsmash. Th MU could become much closer to even at places like FoD or GHZ, but the probability of marth getting one of these stages on a CP is a lot less given theres more relevant stages that ZSS will do better than him on (FD, BF, any big stage like delfinos or DL, PS2, etc)
fox: i wont go into depth much here, as i agree its one of her worst MUs, but the main thing is that she has the tools in neutral such that shes not significantly disadvanted. She has a much easier time getting grabs than 95% of the cast, and her chain grab runs from 20-80% with a guarenteed bair or fair a lot of the time. considering other safe neutral tools like paralyzer and bair, i dont think its so bad that its any more than somewhere between -1 and -2 (60-40 to me).
wolf: your blaster outpaces his, and you have a better DD than him (and higher run speed, and mobility in gerneral). less beefy hitboxes from neutral mean that wolf has a harder time dealing with the gaps in between ZSSs attacks. slightly higher lag on most of wolfs moves (relative to fox at least) contribute to this as well. tether punishes/edge guards arent quite as brutal as fox's, but are still very very good. the difference in neutral imo is what makes this more towards (55-45 on bigger stages, 60-40 on smaller ones imo. definitely not quite -3).
falco: if falco got FD every time, itd be one things. but ZSS is able to contend much better on most of the other maps with platforms, since her mobility around platforms is extremely good. a low crawl helps with lasers too (falco doesnt want to fire them low every time). this adds up to her being able to contend in neutral surprising well, though still being disadvantaged a bit. he has a pretty decent punish for onstage tether jumps, but relatively poor offstage edge guarding means ZSS is going to come back from anything and everything so long as she doesnt get killed outright. falco still wins this for the most part, imo, just a bit more marginally. the quality of the neutral game advantage that falco has isnt enough to make up for the quality of ZSSs punish game on him, such that it would be -3 (70-30). Imo this is the easiest spacie MU. might say its evenish on the largest stages like DL and delfinos. all other stages are at least 55-45. if YS was a thing (its not being put on the stagelist on a lot of tournaments) then hed probably be 60-40 there. FD might be 60-40.
kriby: hes not mobile on the ground, and ZSSs ability to threaten airspace is reallly good. the main thing he has going for him is that he can duck under ZSSs normal grab, which forces her to sometimes use her laggy dash grab. as opposed to shiek however (who also has a crawl, and a dtilt/grab that converts really well) kirby wont get as much from it as you think. a crouching kirby is an immobile kirby, and immobile kirbys get dsmashed. since hes already at a significant mobility disadvantage, i find it unlikely that this trait will allow him to beat ZSS. he does have a good edge guarding game, but compared to other characters, ZSS has some of the best offstage burst movement options to defend against edge guards. doesnt mean she wont get edge guarded, just a bit less than others. overall I cant see kriby being able to garner the conversions he needs often enough to put ZSS a position where he will edge guard her so hard that the MU is in his favor. just cant see it.