There are different rules depending on if you are based in Sweden or in Norway. Please see what applies to you below:
You fill in your income from Hygglo in the income tax return. It is taxed as capital income, which in most cases is 30% (regardless of income from salary). It is currently taxed from the first earned Swedish Krona.
More information can be found at the Swedish Tax Agency: www.skatteverket.se/privat/skatter/
You can earn up to NOK 10,000 tax-free a year.
The Norwegian Tax Administration state the following:
Rental of car and other items.
Applies to you who rent out the car or other items, such as tools, trailers or other belongings. When you also use the item and/or your car privately and gross rental income is a maximum of NOK 10,000 a year, you do not pay tax on the rental. If the rental income exceeds NOK 10,000, the income must be reported in the tax return. It is the same whether the rental takes place privately or through an intermediary (application or online service such as Hygglo).
For more info, see the Tax Administration's page for sharing economy.