Your first sentence:
In case you're living under a rock, this app allows you to ...
would translate into:
Au cas où tu habites dans une grotte : cette application permet de...
And as for the one yout finally sent to your friend:
Quoi ?! Qu'est-ce que ça te coûte de sortir un peu plus souvent, ma petite troglodyte !
I'd say:
Tu ne connais pas ?! Tu habites dans une grotte ou quoi ?
What you sent has 2 things that are a bit weird/not really idiomatic in a casual relationship:
- "Qu'est-ce que ça te coûte de sortir un peu plus souvent" may sound a bit aggressive, and does not match the joking/casual tone you wanted. This expression is used when there's actually something to really blame about.
- "troglodyte" is a funny word but seldom used in a casual chat.
Also note that the whole "ma petite troglodyte" may be seen as flirting... but maybe that was the intention...