"It's a good lark" would mean a good practical joke or fun which is probably not good behaviour. The only relevant possible reference I can find to lurk on Urban Dictionary refers to skateboard slang.

When older (1950s era and earlier) Australians said something was "a good lurk" they meant something was "easy money", or reward for little work...

I think you mean 'it's a good look' It means the appearance of someone 'their 'look' (or how they look, how their appearance is unique and different in a fashion sense) is good. Literally, it would be applied when appreciating someones good clothing style. However, it can be used Ironically - for example, the accidentally spilled wine on themselves - it's a good look (not) or the negative could be applied about a social situation. 'He is only separated from his wife for two weeks and he is on a date with a new woman. It's not a good look.'