The only thing wrong with fleece lined crocs is that the soles aren’t grippy enough. Apart from that (well, and the fact that people look upon those who wear them with pity and derision), they’re perfect. Onesies on adults, however, are probably wrong.
Tell that to the monster in the green/orange one with “Forestry” across the back who walked past me in a Norwegian supermarket
Or the teenagers that literally ran across the shopping centre to look at the rail of them outside one shop….
Crocs — ideal bothy or multi-day footwear IME — bungee to the bike rack, doesn’t matter (much) if they get wet, weigh next to nothing, and the croc-a-like ones cost pennies.