It's not just the ending but also other parts of the narrative that impact impressions of previous games. KotoR 2, despite the limitations of not being able to import KotoR 1 saves, helped give more context and a better understanding of the events of KotoR 1 while ME 3 invalidated much of the main plot of ME 2.
KotoR 2 expanded upon Revan, explaining that he wasn't just a stereotypical power hungry Sith Lord but a visionary who sacrificed himself to the Dark Side to unite the galaxy. Similarly, it added much more to the lore of the Mandalorian War, its effect on the Republic and its part in the True Sith's plan. KotoR 2 added to the experience of KotoR 1.
ME 3 made most of the main plot of ME 2 pointless. When you're playing ME 2, you're thinking or are led to think:
1. Collectors were a major asset to the Reapers and stopping them would seriously hurt the Reapers, right?
Nope, there's nothing in ME 3 to suggest that the destruction of the Collectors was a big setback for the Reapers. In fact, the Reapers arrived just a few months after the Collectors were destroyed.
2. Destroying the Collectors made Shepard an even bigger hero, helped bring more attention to the Reaper threat, brought the different races together, and gave the galaxy a much better chance against the Reapers, right?
Nope, "The Arrival" completely wiped out all the goodwill Shepard got from destroying the Collectors. People continue to ignore Shepard's warnings and remain ignorant of the Reapers. Everyone is still unprepared.
3. This awesome team you gathered against the Collectors will be a huge asset and continue to fight with you against the Reapers, and that gives me an extra incentive to keep them alive, right?
Nope, the whole team disbanded and went their separate ways after "The Arrival" DLC. A few of your team members do some cool stuff but most of them have other stuff to do and don't have time to fight alongside you.
4. Surely, the decision to keep or destroy the Collector base was significant, whether or not Cerberus is still on my side no doubt depends on it, right?
Nope, Cerberus becomes evil. Destroying the Collector base has no real consequences. Keeping it gives you access to a different ending that's mostly same with the "best" ending.