If you’re thinking of putting a comic relief character in your story:

• don't 

if you’re thinking of making a pre-existing/canon character into a comic relief character:

• really don’t

If you’re thinking of making a comic relief character based on harmful stereotypes (fat person obsessed with food, ‘dumb’ blond, etcetc)

• Find a running track
• cover the whole thing in legos
• run a few laps barefoot