Is Camp Lakebottom truly a rip-off to Gravity Falls?

No.

Copied from here (adding modifications): Well:

1. Camp Lakebottom

2. Phineas and Ferb

3 Gravity Falls

There are A LOT of differences between GF and CLB. 

 1 . There are two children in GF, but siblings, while in CLB, 3, but friends.

2. Dipper and Mabel are in a  TOWN , but (I don't remember that boy's name with a semi-brown hair and a blue T-shirt), Gretchen and Squirt are in a CAMP .

3. There are three special people in both shows, Stan is the founder of the Mystery Shack, Wendy is the janitress and Soos is the handiman. The three other people in that camp, are counselors (I only remember Sawyer, that zombie dude, which is also a handiman). One is a chef, and that sasquatch guy, I don't know.

4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15. Moar.

Kay.
Well, problem solved, every pony body.