Not too bad. Quick things I noticed:
1. Early % Uthrow vs Marth is fine. However, your best option after doing this is to follow him with a short hop, not a full hop. Full hopping means regardless of what you choose to throw out or wait for, he can airdodge to the ground too quickly and you can't cover it in time. If you SH, you maintain a fair amount of space while retaining the option of going further up to hit him. If he airdodges early out of habit or fear, its easier to hit him this way.
2. You had some trouble punishing him OOS in certain spots with aerials. Your best aerial to punish Marth with OOS in most circumstances is Nair. Quickest option that covers both sides and can also lead into combos/mixups with the double hit. Fair and Bair depending on the circumstance are fine too, but try to skip Uair unless he does something silly like a full hop approaching Fair REALLY close by and you know it will hit. You had Shield Drop Fsmash/PS Fsmash down pretty well, just something to note if you want to punish some of his retreating aerials and mixups better.
3. If Marth has used his DJ and he's moving onstage, 80% of the time he's gonna airdodge due to the threat of clap/fart. Try to think of where he will weave to land, and focus on zoning that area instead of zoning him higher up in the air. In general it's much easier to punish Marth when he's airborne and you're closer to the ground. If you are both airborne or you are rising quickly due to DJ while he is falling, it's a coin toss and a lot of times his airdodge will be good enough to get down safely. Where as if you zone the area he will probably land near, he has a lot of stuff to worry about.
4. Bite is a hard move to use vs Marth. I tend to nearly skip it 90% of the time because I don't want to be lagging and eat anything dangerous from Marth regardless of %. If you can make it work, more power to you. I dislike the move in that MU and refrain from using it much even when he's pressured on the edge.
5. If you weak hit Fair Marth, expect a Fair to the face right back if he's not at 50+ damage. You HAVE to strong hit Fair early on if you want the time or safety to retreat. Weak Fair can punish mostly anything but the most extreme retreating attack or spacing, but it tends to not be worth it early on if you take more damage than he does in the trade.
1. Early % Uthrow vs Marth is fine. However, your best option after doing this is to follow him with a short hop, not a full hop. Full hopping means regardless of what you choose to throw out or wait for, he can airdodge to the ground too quickly and you can't cover it in time. If you SH, you maintain a fair amount of space while retaining the option of going further up to hit him. If he airdodges early out of habit or fear, its easier to hit him this way.
2. You had some trouble punishing him OOS in certain spots with aerials. Your best aerial to punish Marth with OOS in most circumstances is Nair. Quickest option that covers both sides and can also lead into combos/mixups with the double hit. Fair and Bair depending on the circumstance are fine too, but try to skip Uair unless he does something silly like a full hop approaching Fair REALLY close by and you know it will hit. You had Shield Drop Fsmash/PS Fsmash down pretty well, just something to note if you want to punish some of his retreating aerials and mixups better.
3. If Marth has used his DJ and he's moving onstage, 80% of the time he's gonna airdodge due to the threat of clap/fart. Try to think of where he will weave to land, and focus on zoning that area instead of zoning him higher up in the air. In general it's much easier to punish Marth when he's airborne and you're closer to the ground. If you are both airborne or you are rising quickly due to DJ while he is falling, it's a coin toss and a lot of times his airdodge will be good enough to get down safely. Where as if you zone the area he will probably land near, he has a lot of stuff to worry about.
4. Bite is a hard move to use vs Marth. I tend to nearly skip it 90% of the time because I don't want to be lagging and eat anything dangerous from Marth regardless of %. If you can make it work, more power to you. I dislike the move in that MU and refrain from using it much even when he's pressured on the edge.
5. If you weak hit Fair Marth, expect a Fair to the face right back if he's not at 50+ damage. You HAVE to strong hit Fair early on if you want the time or safety to retreat. Weak Fair can punish mostly anything but the most extreme retreating attack or spacing, but it tends to not be worth it early on if you take more damage than he does in the trade.
$10 MM And I did it (mostly) without using the chaingrab
, vs Will 
