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As well-thought out as the answer before mine is, here's the scoop:

The Oncler in the newest adaptation was once heavily meme'd as a "Tumblr sexy-man"– an internet bad-boy that folks would write edgy fanfiction about and make thirsty art depicting them, among other things. It was era that people who had that phase look back on and cringe. It is not something that people are proud to remember about themselves.

It was clearly unintentional by OP but once they saw the resemblance, they were filled with deep regret considering the history of the Tumblr-fied-era Onceler.

So comparing OP to the Onceler is bringing up feelings of adjacent cringe, as being compared to a Tumblr-phase-animated-fanfiction-sexyboi-twink really makes your brain scream "OH PLEASE NO" if you remember "those times".