Harvey Birdman: Attorney at Law wasn't setup as an origin story. Harvey was no longer a superhero and now practiced law. Velma tried to create an origin story for a well known popular character, overwriting existing literature.

Velma might have been better received if they had framed it as her post-mystery incorporated life, ie what's Velma doing now that she's left the crew. That would have given them much more room to work with.

You can see this in other spin-offs like Harley Quinn which is set in her post-joker life.