To me, Mass Effect ends about six hours into Mass Effect 1, when I get bored and wander off to play something else. I've had ears and eyes long enough to know that people were upset about the ending of Mass Effect 3 however, even after BioWare tweaked it. The original Mass Effect 3 had various "Happy Ending Mods" to change the close of Commander Shepard's story, and now Mass Effect Legendary Edition does, too.
Audemus' Happy Ending Mod (AHEM) is available from Nexus Mods, as spotted by PCGN and alters cutscenes, ending slides, and codex entries among other elements to give a more upbeat spin to the story. I won't mention the main detail it changes in case you, like me, haven't played it and don't want spoilers, but you can click the Nexus link and find out more if you wish.
Here's a video of the new ending in action:
The new ending will take into account the decisions you've made across the course of the game, changing based on the number of war assets you have collected. Specifically, the ending created here is similar to "Ending B" from JohnP's Alternate Mass Effect Happy Ending Mod for ME3.
I like a good sad ending, but I also respect anyone who consumes a bit of fiction, doesn't like an element of it, and decides to create what they want themselves. Whether that's via modding a game or posting some fanfic to AO3, fair play to you. Kudos to people who make something, versus just complaining on the internet or harrassing the original creators.
If you've yet to play Mass Effect in any form, the Legendary Edition - which remasters the entire trilogy - came to Game Pass just a couple of days ago.