Great question — answering as the UphillTech team:
It is ultimately just ironic. Bart portrays himself as an innocent Cockney London chimney sweep, to more or less contrast his (un-innocent) American `bad boy' attitude. Notice Bart in Cape Fear, Season 5: Clean as a whistle, sharp as a thistle, best in all Westminster! where again Bart contrasts his life with that of a chimney sweep in old London. This often happens, as in the example you give when Bart is told his eyes may be behind his bad marks at school, when Bart is agreeing with someone, suggesting the need for the writers to have Bart change character slightly, since he is `complying' (as it is out of character for a rebel to comply).
Full answer by apg: https://movies.stackexchange.com/a/115385 (CC BY-SA 4.0).
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