I can only speak for me, anon-chan, so my answer may differ than anyone else’s.
I ship GeekChic because the possibilities for these two being romantically involved are endlessly interesting. There is very little one-on-one interaction with Serena and Clemont in Pokeani canon (mostly due to the writing being dedicated to Serena’s crush on/relationship with Ash) but - for all their differences - they actually have a fair amount in common.
They both have mild anxiety/bashfulness, they both love cooking, they both have an interest in creating unique/helpful/inspirational things. Serena sews and bakes, Clemont invents and cooks.
To be fair, I’m not Serena’s biggest fan. She was introduced poorly, written poorly. I don’t bash her because I know she’s intended to be a good character/person, but she has been handled so - well - poorly that I don’t really know what to think of her. Even now that she has a goal, she still seems highly one-dimensional to me. She has interest only in performing, baking, pretty clothes, and getting Ash’s attention. I just… I like girls with a little more “meat” on their character bio.
But I looooove Clemont. I want him happy. And I think Serena and her semi-shared interests, experiences, and behavior would fit him well. I think her different interests, experiences, and behaviors would give him a nurturing opportunity to grow. And I think Clemont could offer her the same things.