You can set popular attribute in sort by option by below way,
First create attribute popular from Admin panel,
Stores -> Attributes -> Products
Create popular attribute with text field.
Go to Storefront properties tab in attribute page,
Set field value Used for Sorting in Product Listing to yes
Save Attribute.
Now set popular attribute into Attribute set.
Stores -> Attributes -> Attribute set
Save your attribute popular into attribute set.
Now go to product page,
Set Popular attribute field value like 1,2 else you want.
Save product.
Open listing page and you can display all those product with lowest value first for popular ascending order.
if issue solved, click kudos/accept as solutions.
