In this review Jonathan Jones is complaining that the artist's work is quite jolly, colourful and idiosyncratic, but lacks profundity or any strong emotion (e.g. rage).
For the meaning of "whimsy" -- which is not covered at all adequately by putzig or altmodisch -- it's best to consult monolingual dictionaries, by searching for "whimsy meaning".
unusual, funny, and pleasant ideas or qualities:
Add a touch of whimsy to your room with these cat sculptures.
The film combines whimsy and tragedy in equal measure.
something that is intended to be strange and humorous but in fact has little real meaning or value:
Personally I've always considered mime to be a lot of whimsy.
Playfully quaint or fanciful behaviour or humour.
‘the film is an awkward blend of whimsy and moralizing’
Whimsy is behaviour which is unusual, playful, and unpredictable, rather than having any serious reason or purpose behind it.
... 2 : the quality or state of being whimsical or fanciful
the designer's new line showed a touch of whimsy
3 : a fanciful or fantastic device, object, or creation especially in writing or art