Iron Gate mentioned recently they've been getting many questions on their stance of modding Valheim, so they wrote a post to give their thoughts including the subject of paid mods.
Valheim doesn't actually have official modding support but that's never really stopped anyone has it? Mods help keep games alive for years after developer move on, so it's pretty much always a good idea to support it.
In their post on their official site (and also Steam), Iron Gate mentioned how they're "happy" to see people make mods and they find it "flattering" people wish to do it, however by not having official support they're not giving any kind of guarantees that mods will work between releases - so you do so at your own risk.
On the subject of paid mods though? They're not particularly happy about that as they do "not condone locking modded content behind a paywall" and they feel that charging money for a mod "is against the creative and open spirit of modding itself, and therefore we urge all mod authors to make their mods freely available to all who want to play them". They have no problem with donations but they don't want to see any part of a mod behind a paywall.
Additionally, they're asking mod creators to ensure they make it clear that their work is unofficial.
Every developer will likely feel different about this, as will a fair amount of gamers. How do you feel about paid mods?