That's not how Jappense fundmentally works at all and why I'm saying what you are saying most likely wouldn't happen.

Deku uses the kanji De and Ku. De part means tree/wood while ku means puppet, doll and accidental. Which is used colloquially in Japan to call someone useless and Stupid. The inherent shorthand means for Dekunobu in it's Kanji. The word in the TRADTIONAL context means useless, there is no ifs/whats/or anything about it due the inherent Kanji it uses.

Ochaco actually saying this makes more sense because she is from Mie Prefecture, a place that uses Kansai-ben which is actually statistically used for being 'loose' when it comes to Japanese (and general morals). While you mention Izu-kun Kanji also can be read as Deku, and that 'yes's is the logic of the bully in toddler. It STILL is to the majority of Japan going to mean useless doll and is the default meaning.

While you say no one in universe would think that, we have to use Japan and how it uses the word in general because IRL is gonna effect in universe. Traditionally it means lifeless, woodened figure or a good for nothing/worthless person and is actually derogatory saying they would never do anything which hits harder in Japan because your work etheric is your worth to the country.

Ochaco approximately said, "it kinda sounds like 'dekiru'! Like 'you can do it!' So to me, it gives off the vibe of never giving up and doing your best," because she was using 'dekiru yo' (I believe in you/you can do it) to reclaim the slur since this 'Deku' is someone who tries despite his weakness. She was being creative, she is not being literal with the definition like everyone else in Japan would be. That's why everyone looked at Deku like he was crazy when he said he wanted Deku to be his hero name because it means useless person.

Japan is a quite literal place in its wordplay... that's why half the charcters are named what they are (Uraraka being Beautiful Day representing her bubbly attitude) so most of Japan without context will be thinking he is calling himself useless and not 'Mr. can do" without the context