Well there is:
Camping Continental (on the north side of town) https://www.camping-continental.it/en/
Camping Spiaggia (a little bit to the south of Mandello) Camping Spiaggia - Abbadia Lariana - Welcome
The town used to open the “Lido” (lakeshore park) for free camping, but it has next to no facilities and was overcrowded 11 years ago.
I 2021 the town had planned to use the nearby football field for camping, but you needed to book with the town.