I didn't say Serena doesn't notice, more like she doesn't have eyes for the people who do show interest in her.
Speaking of Brock-
What if say Brock came back. He gets to know about Serena's crush on Ash. What will he do to help Amourshipping?
DP fans confirm this. There was this kid in DP, I think Kenny was his name, who had a crush on Dawn. I remember Brock giving love advice to that kid. So I imagine Brock as the ultimate shipper in deck in pokemon due to his "experience" with love.
I don't remember if Brock ever directly helped Kenny other than suggesting a battle with Ash because there was a love triangle that wasn't a love triangle going on there. Wait, nope, Brock wasn't involved at all with that. If anything, he'd cry that Ash has a girl canonically take an interest in him so ardently.