I think it's good to be passive in neutral about 70% of the time, because Marth's ability to cover options, and react and punish is very good. Obviously, this doesn't apply to bad characters who cannot deal with pressure, or punish the attempt of a Fair.
However, once Marth lands a hit, his ability to follow up and make aggressive reads is very good. Marth's ability to create long strings of reads and traps, to deal lots of damage once obtaining positional advantage is very good, and should be exploited.
Against D3, walk, do not fast fall, use Nair to zone, and learn to powershield Ftilt on reaction, since it gives you free Dancing Blade punishes. Also learn how to exploit D3's recovery. Once he has no jumps left, he must use his UpB, which does not sweetspot the edge, and allows you to hit D3 off the stage over and over. Also acknowledge that footstooling D3 is often a better option than landing near him if it can't be avoided otherwise.
Rob's spot dodge doesn't reliably beat Dancing Blade, so learn to increase the pacing to defeat spotdodges. Otherwise, vs. ROB is just a match of slow conditioning. If the ROB favors Ftilt, use Dashing shield to punish. If the Rob favors grab, approach with other options (DB, Fair). Zone him with Nair, as it stabs his shield easily, kills well, and sends him fair offstage. In this MU, you mainly want to just get ROB off the stage, and then ledgetrap or juggle, just like every other character. Don't fish the kill, there are plenty of opportunities to
If any character's dodges are beating you, that just means you need to be more patient. ROB has a standard spot dodge, but Falco, Pikachu, Yoshi, Link, and Toon Link all have broken spotdodges that only have 2 frames of vulnerability at the end (and one frame in the beginning). I don't really try to punish these, as even Dancing Blade has a large chance of failing to hit such a small window.
Other than that, you simply have to acknowledge that you need to anticipate dodges as an option, and punish accordingly. It's one of the main reasons that "pressuring your opponents within his tipper zone" is not really the unbreakable strategy that it looks like on paper. Dodges are pretty effective vs. Marth. Spotdodges beat most of his offensive options (not Dancing Blade) and roll beats Dancing Blade, so it's unsafe to play without anticipating the use of dodges, but patience will almost always defeat a dodge.