No, it does not matter where your JPEG comes from. Editing a JPEG degrades its quality.

The reason that JPEG is not suited to editing has to do with the way the data is saved in JPEGs, not whether the camera produced the JPEG or Lightroom did. I suggest you read the many excellent answers on this site that explain why raw is better than JPEG for editing.

In your test, you found that the quality did not degrade when editing the JPEG. I suspect that one of two things is the problem:

1. Your edit is not substantial enough and you are not looking close enough. A very few number of edits can be made without quality loss (some scaling and rotation by 90/180 degrees) and many more type of edits can be made with only a bit of information loss.

   Example: You crop the image. Most of the time this will be a lossy edit. But the quality loss will be small and you might not notice it. However, then you also change brightness a bit, then you rotate by a few degrees. All these small losses accumulate and in the end when you print or look at the image on a high-resolution screen you can see the difference.

2. Lightroom is behaving differently to what you expect. Lightroom is first and foremost an image database. It keeps track of your images and also what edits you did to them. It's misleading to think that you "open" an image, edit it, and then "save" it as JPEG. You only "import" and "export" the image.

   Are you sure that when you exported the image as JPEG, you did not include it in the Catalog, then later imported the image to the catalog? On export there is a small check box that let's you "include the exported image in the catalog". That means Lightroom knows that the exported image is just one version of the original raw. If you now edit the image some more, you don't do so on the JPEG data but on the raw with some edits already applied.

Edit

In response to one comment below: When I talk about "editing a JPEG degrades its quality", I mean the most common scenario where a user opens a JPEG in her favourite editing programm, does something to it (i.e. rotate by 3 degrees and crop to fit) and then saves as a JPEG again.

• There is no information loss when opening a JPEG. At this stage a decompression occurs and the JPEG data are converted into a RGB format that can be displayed on the screen. Yes, JPEG uses a lossy compression but this loss occurs when compressing not when decompressing.

• The information loss occurs at the stage of the edit (for a large subset of edits), this can be mitigated by upscaling, or moving to more than 8bit channel representation. (Note that a rotation would also cause a loss when done in raw, the difference is that in raw Lightroom always starts from the raw so losses of various edits don't accumulate.)

• There is also information loss when compressing/saving the edited image as JPEG. From the two sources of loss, I suspect that compressing causes more loss than editing but this test has many variables, so can't be answered here. Have a look here for a good visualisation to what does occur.

• The comment suggests that editing a JPEG can be done without quality loss when saving in a lossless format like PNG. This is only sometimes true (see point above) but more importantly, it is also irrelevant: If the reader knows a bit about the intricacies of JPEG, the question can be answered immediately. If, however, the reader does not know these details about JPEG then she is unlikely to save in PNG.

Practical relevance: If you edit in JPEG and only do one save you only lose a bit of quality. If you do so, it's up to you to decide if you can live with that. Most of the time a single edit will not be noticeable. But I wrote that as part of my original answer anyway.