I can comprehend why Serena's interaction with Ash was lackluster for some. For starters, they had nothing common in terms of career goals. At least contests involved a battling round and Misty/Iris were trainers just like Ash. Serena arguably interacted with the most matured version of Ash which prevented moments of playful teasing and bantering between them. In real life many people act shy in front of their crushes at the beginning. Plus, Serena got around 50 episodes less than Dawn/May in the anime. Ash's follow up interactions with Serena would have been interesting to see had the kiss happened a few episodes earlier, probably that would have strengthened their bond. Still, I think the support Ash offered to Serena was very significant and easily comparable to what Ash did for his other female companions.
I think Serena's impact on Ash is somewhat underrated. Surely, Ash would have gotten over his depression after the Wulfric loss all by himself. But, we would never know how much longer had it taken him to do so if not for Serena's involvement. Serena was the driving force behind the motivation Ash found after the only argument he ever had with her. In the same episode later he perfected Ash-Greninja which was probably the most important reason behind his best ever finish in a pokemon league.
I think Serena's impact on Ash is somewhat underrated. Surely, Ash would have gotten over his depression after the Wulfric loss all by himself. But, we would never know how much longer had it taken him to do so if not for Serena's involvement. Serena was the driving force behind the motivation Ash found after the only argument he ever had with her. In the same episode later he perfected Ash-Greninja which was probably the most important reason behind his best ever finish in a pokemon league.