Not familiar with that match up yet. I can imagine the frustration though. She not only has raw speed, but very good mobility too. Her stunning tools and our size are not a winning combo for us. Since i'm not qualified to give you specific MU pointers, i'll just fill some air with general knowledge i've learned from Bowser.
Some of Bowser's more defensive moves (I think) are
Jab: One of Bowser's fastest moves. Nothing special, but still useful for keep away. 1st jab can connect to a side b or tilts if done properly. If you're comfortable with wavedashing, jab twice and WD towards the opponent for another jab or two or whatever you'd like.
Dtilt: One-two attack for 22% total. Disjointed and strong. Has the benefit of a little armor (though it does not seem to gain the entire effect of crawl armor). Also, you can turn around while crouching by holding the opposite direction after you put out the first dtilt hit. Sometimes good enough to land a Bair. You can always Bowser Bomb (Jump Cancel) to change it up and hopefully soak up a jab or dash attack as the bomb's jump squat has (weak) red armor.
Crawl Armor: Not a move per say, but unique and essential to Bowser. Crouching instantly gives him light(questionable term in this case, if it hasn't been relabeled yet) armor that allows you to take projectiles and some damn strong hits at 0-low percents. The effect minimizes, but does not disappear, to an extent as rising damage continues. This should actually be able to take a good grip of ZSS's moves as she uses weak multi-hit moves frequently, and just generally weaker.
Nair: One of three non-smash attacks with red armor AND the only one that's L-cancelable. AWESOME. I think this is an overlooked move, and I actually like this one more than Fair (personally. Not so much as an option replacement). Red armor with 13% and no sourspot (I think). It can beat a good amount of aerial attacks. The blessing of red armor alone lets you shut down even more general options, and not just aerials. Getup attacks? SQUASHED. Recovering? WALLED. Attacking above you? INTERCEPTED. Use it in junction with Bowser Bomb Jump Cancel to turn yourself into a cannon ball. Drop it hard with some SHFFLS. You'll see what I mean.
Up-B: You know this one. I know this one. Our grannys know this one. Great for offense, defense, killing, retreating, and I wouldnt rule out mincing onions either. It's a move that you can get kind of creative with. Out of Shield is quintessential and lifesaving. Also, use it instead of a dash attack to get past the opponent and onto his/her ledge since Up-B, compared to dash attack, has a longer duration AND multi-hitboxes that can better disrupt, or can just straight-up knock them out. Use it instead of an aerial at low percents to deal maximum damage, because If you can land the strong AND weak hitboxes of the entire move, then that's a quick 35%. Personally, I try to stick with a single direction when i'm grounded in order to minimize whiffing and maximize edgehogging.
Dsmash: Kind of situational, but really awesome. One of Bowser's few multi-hit moves, and it has generous amounts of red armor, though each hit is weak so i'm not sure how good that armor really is. It'll still disrupt, that's for sure. Mostly used as a surprise, or trap. Decent for edgeguarding at low/really high percents as it lands 28% total and can kill above 150%. NOTE: Dsmash actually has a gnarly disjointed hitbox behind Bowser that WILL get unfamiliar people by surprise. Personally, I actually approach with it from time to time. If you're comfortable with wavedashing, you can WD with your back towards the enemy and time that disjointed hitbox. It's great every time.
Flame Breath: First is first-this move does not suck. It is actually quite useful. It's not the greatest iteration in any sense, but damnit if I don't use this every match. Minimal knockback with considerable end lag, but has instant startup if landed with. A good shield pressure option and your best long-range spacer, though that doesn't say much unfortunately. It's all about spacing with this one. Learn to time it. OH! B-Reversals! Learn this too!
IMPORTANT: Fair and Bair end way quicker in 2.6b. To the point of autocancelling reliably. This is fantastic news for Bowser's offense and defense. It allows him to do this: You can accompany each full jump with a Fair or Bair, and still have enough time in the air to land with Nair (for opponents under you), Flame Cancel (for opponents out of immediate reach), or waveland for a safer retreat. There's a bit of room for creativity with this technique so you can change it up depending on your offensive or defensive needs.
I got carried away here, but i'm hoping at least ONE sentence made SOMETHING better. PROST!