The pejorative most often thrown Robin Williams’s way was “sentimental”. Dead Poet’s Society is sentimental; Good Will Hunting was sentimental. The Fisher King is a hugely sentimental confection (although Terry Gilliam has quite a lot to do with that).
Often what Williams was portraying was deep loneliness; and lonely people have sentimental turns. Commonplace words like “love” and “acceptance” get capitalised, become the Love and Acceptance on which entire worlds pivot. You make a big deal about these things because they’re difficult, elusive, never quite seeming to work out; redemption can happen in fantasy at least (although the fantasy itself is not redeemable).
I think his work will bear rewatching with an eye to its strangeness, its estrangement. Jumanji, already a weird and troubled film, gets considerably weirder when Williams emerges from the box, a lost child grown adult. Even the aggravated kookiness of Mork and Mindy jaunts along over an ostinato of disconnection and anxiety (Mork is an outcast from Ork, a misfit wherever he finds himself). It is not easy to live in this world.
Can it really be true, as wikipedia reports, that the landlord of Fred and Mindy’s music store was named “Arnold Wanker”?