[Rhod the Red]

Radagast's mission wasn't to commune with the birds and bees

Tolkein wrote a brief insight to the council of the Valar that we can look at. "each Istar were chosen by each Valar for his innate characteristics", which points to awareness of what style of efforts each Istar would make and lives they would lead. It implies Yavanna chose Aiwendil (Radagast) because of his love of wild creatures, whioch no doubt helped in the cause against the Dark Powers.

Tolkein gets grey over time after his initial writing of him. So it's sort of speculative whether he outright 'failed'.

In the books we only have Saruman pouring scorn over him (even when he follows Saruman's & Gandalf's orders precisely), no one else does. Gandalf is very praiseworthy of him (my emphasis).

Radagast is, of course, a worthy wizard, a master of shapes and changes of hue; and he has much lore of herbs and beasts, and birds are especially his friends.

The conclusion of a section of the Tolkein Gateway page on him mirriors my conclusion. That he plays an obscure role in the battle against the Dark Powers, not a lazy one.