it´s absolutely correct to say "Ich habe bemerkt, dass.." in the sense of "I have noticed"
The other translation is also correct: " During the discussion I mentioned that XY is an idiot"-
Während der Diskussion habe ich bemerkt, dass XY...(this uses "Bemerken" as "mention sth", but still in the german sentence it could mean both, thats why we prefer to say "eine Bemerkung machen"which gives no doubt about what you mean...)
It´s really difficult, because the sentence quoted above could also mean "During the discussion, I noticed, that XY is an..." so it always depends on the context....
To be on the safe side: Während der Diskussion habe ich die Bemerkung gemacht, dass XY ...-
During the discussion I mentioned, that....
"merken" is used more emotionally: Ich merke, dass du schlechte Laune hast - I feel. that your are in a bad mood, "Ich merke, dass ich älter werde - I feel, that I grow old"
And last but not least: "merken" means also "remember"...
"Das werde ich mir merken" - "I´ll remember that!"
..phew, what a language..;-))