Mona Lisa worked well because she actually had a personality, and wasn't just there for Raph to swoon over, or vice versa, her being a Raph fangirl or something.
Looking back she was extremely well written for a potential love interest, it never felt forced and neither Raph nor Mona Lisa instantly started falling for each other. Yeah I did wish she got at least one more appearance.
The villain of the ep was pretty lame though, even if I liked his purple monsters or whatever the hell those things were.
