It's very informal, so you wouldn't want to use it in academic writing. We put decorations all around the yard. We decorated it thoroughly. Just read the rest of the paragraph after it, and the decorations are described in detail.

‘All up’ has several meanings related to conveying completeness or in total. To me it sounds a bit childish here, as it should. We’re out of milk. I used it all up. (Completely) I did well all up. (Counting everything. Taking into account all factors) It cost me $20k all up. (Including everything) In your example it means ‘completely’. It conveys the informal and youthful nature of the dialogue.

😆 That’s very native, informal, and “incorrect” according to rules. But I use it, and I hear it often. The narrator was just expressing that they decorated the yard “really well”; they put effort into it.