Did Darth Maul Become Less Powerful After His Duel With Obi-Wan?

No, he got stronger. The difference between Darth Maul and Darth Vader was never the wounds, because losing limbs did not reduce a Force user's potential. Any drop in power came from the mental failings of the injured, and could be overcome with willpower. Vader never made that recovery. Maul was arrogant enough that losing half his body barely dented his confidence, so his use of the dark side went untouched. (Dark Lord: The Rise of Darth Vader; The Clone Wars, "Revival", "The Lawless")

The claim that started the debate#

The idea that Vader never reached his potential because he lost too much living flesh on Mustafar is widespread, and it has a source inside the fiction. The Sith Lady Lumiya, who took extensive prosthetics of her own after crashing her starfighter in a dogfight with Luke Skywalker, believed her cybernetic implants limited her Force potential, so much that she gave up hope of ruling as a Sith herself and looked for someone else to back. (Legacy of the Force: Sacrifice)

Note whose claim it is. Lumiya was a heavily augmented Sith with a personal stake in believing her ceiling was hardware rather than herself. Others thought otherwise, including Vader's own master. (Legacy of the Force: Sacrifice; Dark Lord: The Rise of Darth Vader)

The Legends record mostly comes down against Lumiya. Losing limbs did not have to reduce a Force user. The clearest case is Vader, whose limits are put down to his state of mind rather than to the metal that replaced his flesh. (Dark Lord: The Rise of Darth Vader)

The one real physical limit#

There is exactly one exception, and it is narrow. (Book of Sith: Secrets from the Dark Side)

Force lightning almost always needed a living body to channel it. That is why Vader does not throw lightning: any attempt risked shorting out the life-support systems that kept him alive. It is not that the dark side had abandoned him, it is that the hands are not his. (Book of Sith: Secrets from the Dark Side; Star Wars: Revenge of the Sith: The Visual Dictionary)

Everything else was still on the table. Even after Vader lost a great deal of his organic body, he kept an extraordinarily high midi-chlorian count. He could overcome any apparent loss of Force ability, with the lightning exception, if he could overcome his mental limitations. (Star Wars: The Complete Visual Dictionary; Dark Lord: The Rise of Darth Vader)

That last clause is the whole ballgame. (Dark Lord: The Rise of Darth Vader)

Palpatine's own diagnosis#

The best evidence that the block was psychological comes from the person with the most reason to care. (Dark Lord: The Rise of Darth Vader)

Sidious had waited decades for Anakin Skywalker and was robbed of his perfect apprentice when Skywalker's power was severely diminished by his injuries on Mustafar. He did not conclude that Vader was permanently capped. He believed Vader might still be capable of achieving his full potential with a complete psychological recovery, while keeping an eye out for a Force sensitive who could replace his hobbled disciple. (Dark Lord: The Rise of Darth Vader)

Both attempts at a replacement failed. Galen Marek chose to sacrifice himself for the Rebel Alliance, and Luke Skywalker put his effort into redeeming his father instead of killing and replacing him. Sidious also knew finding another apprentice as powerful as Vader was extremely unlikely. (Star Wars: The Force Unleashed; Return of the Jedi)

So the Emperor's own reading of Vader is that the ceiling was in his head, and that if he ever got past it he would be everything he was supposed to be. He never did. (Dark Lord: The Rise of Darth Vader; Return of the Jedi)

Maul had the opposite problem#

Maul's injury was, if anything, worse. Obi-Wan Kenobi cut him in half on Naboo and everyone involved assumed that was the end of it. (The Phantom Menace)

What followed is one of the strangest survival stories in the setting. He spent years on Lotho Minor in near-madness, and fashioned a crude substitute lower half out of scrap, a prosthetic that resembled an arachnid more than any part of a Dathomirian. His brother Savage Opress eventually found him and took him back to Dathomir, where Mother Talzin gave him proper cybernetic legs. Later, Death Watch under Pre Vizsla had them replaced with mechanical legs that restored him to his original height. (The Clone Wars, "Brothers", "Revenge", "Eminence")

The point is what did not happen. There is no period where Maul is diminished in the Force. He came back and built the Shadow Collective, took Mandalore, killed Vizsla with his own weapon and claimed the Darksaber, and fought Sidious himself. He was more dangerous after the injury than before it. (The Clone Wars, "Eminence", "Shades of Reason", "The Lawless")

The reason is temperament. Maul was extraordinarily arrogant and utterly confident in his own abilities, and that confidence meant the injuries never touched his use of the dark side. He did not interpret being cut in half as evidence that he was less than he was. He interpreted it as something to avenge. (The Clone Wars, "Revenge", "The Lawless")

Vader looked at his own body and saw a monument to his failure. Maul looked at his and saw a debt owed to him. (Dark Lord: The Rise of Darth Vader; The Clone Wars, "Revenge")

The takeaway#

Two Sith apprentices, the same enemy, comparable mutilations, opposite outcomes. The mechanic that explains it is not physical at all. (The Phantom Menace; Revenge of the Sith)

Prosthetics only take away what you decide they take away, with the single exception of powers that need living flesh to leave the body. Vader spent twenty years believing he was a ruin, and so he was. Maul spent the same years believing he was owed a galaxy, and grew into something closer to it. (Dark Lord: The Rise of Darth Vader; Book of Sith: Secrets from the Dark Side)

This is all Legends framing, drawn from the sourcebooks and novels that filled in how the Force actually works. The current canon has never contradicted it, and the character trajectories in The Clone Wars and Rebels line up neatly with it. (The Clone Wars, "Revival"; Rebels, "Twilight of the Apprentice", "Twin Suns")