Thoughts on why tumblr more likely to like Tenna but hate Flowery:
-Tenna was (possibly?) unaware that the Knight wanted to kidnap Toriel and did not think he was going to have to attack them. Flowery was very clearly warned that the Knight wanted to kidnap Asgore but insisted on trying to keep him in the Dark World anyway out of overconfidence that his fake inflated stats would be enough to defeat the Knight if they showed up.
-Tenna tried to get us to go along with his games through guilt-trips and a genuine desire to spend time with us. Flowery tried to get us to do what he wanted through emotional manipulation and acting like he knew better than us. (not saying guilt-tripping is good but at least Tenna didn't insult our core beliefs)
-Even when Flowery is saying things that might be true, he does so in the most condescending way possible that makes you want to disagree with him out of principal.
-Tenna was obviously flawed and everyone in his world acknowledged he has issues which made him more sympathetic. Everyone in Flowery's world acted like he was the greatest person ever who could do no wrong even though his method was also flawed and we were the only ones who could see that. (this also applies to Rouxls btw, pathetic man nobody respects is more fun)
-Probably a major one: Tenna did not hit on our mom. Tenna did not tell our mom she'd be better off without her Light World life and family, nor did he tell us our mom would be better off if she never saw us again. If Tenna was trying to act like Toriel's new Asgore while the real Asgore was hurt and possibly dying it would have felt a lot different. Yes in Asgore's case it was "best friend who he possibly had a crush on" and not "ex-partner he has kids with" but point still stands, Asgore's actual best friend was possibly dying while this flower guy claiming to be his real best friend kept him trapped and he had no idea and probably still has no idea something happened to Rudy because he got Knight-napped.
-Flowery knew damn well that their Dark World couldn't last forever and that as flowers being in a Dark World too long would literally kill them but he still acted like this wasn't the case and that his Dark World and it's inhabitants should be considered more important than our (naturally created so doesn't affect Roaring and immune to Knight) Dark World with our friends from at least four other Dark Worlds now.
-Misgendering Ralsei and Susie as humans and not bothering to correct anyone else either even though he should know Kris is the only human and the other two are monster/monster-adjacent.
-Flowery is too pretty and hyped up to be a tumblr sexyman, tumblr sexymans need at least a little sad wet cat/obnoxious pathetic blorbo/not conventionally attractive element to them and he has none of that.
-Tenna's "death" was shocking and out of nowhere. Flowery's death felt bad but also he kinda brought it on himself because Ralsei warned him not to underestimate the Knight and he ignored them.
-At least Flowey admitted he was an asshole