I am a firm believer that Stan and Mabel are not the "dumber" twins. They just aren't as interesting in 'nerd' stuff like Ford and Dipper are.
Stan was able to rebuild like 90% of the portal with only the 1 of the journals, most likely having to teach himself advanced mathematics and scientific theories to even understand what the thing is. It barely took him a few weeks to get the portal up and running after getting the other two journals. Not to mention how he was able to create a business and keep people entertained at his day job. The man is smart.
Mabel is able to create sweaters at the drop of a hat. If you don't know anything about knitting, it takes a lot of math and planning to be able to make a sweater, and Mabel is constantly making them for everyone around her. She made a full 2 act puppet shows with musical numbers and sets and well made puppets in like a week. She may not be interested in the 'nerd' stuff like Dipper, but she's creative and thinks in innovative ways.
Both of them are amazingly charismatic. They can make friends and bond with those around them easily, whether it be for business purposes like Stan or simply childlike positivity like Mabel.
They aren't dumb.