From the help files. There is a setting to also import jpg files in Edit/Preferences:
Treat JPEG Files Next To Raw Files As Separate Photos
For photographers who capture raw + JPEG photos on their cameras. Selecting this option imports the JPEG as a standalone photo. If selected, both the raw and the JPEG files are visible and can be edited in Lightroom. If deselected, Lightroom treats the duplicate JPEG as a sidecar file, and the raw file appears with the raw file extension and +jpg.