Learn English – A verb phrase that describes someone that doesn’t pay their debt

Reasonably unformally, but not vulgar, is flake (noun) or flakey (adj.) for somebody who is unreliable.

Oaf means someone who is clumsy and foolish, but doesn't say anything about intent or how others perceive the person. A lovable oaf is more specifically what you mean, but of course isn't a single word.

Come to think of it, buffoon is really close, but maybe a bit more insulting than your intent.