What I love about Waluigi is just how dumb his design is. I mean, nothing about the design of the character makes sense.
Flipping the 'M' in Mario upside down made sense. Wario is the antithesis of Mario, he's the flip side of the coin so to speak, so flipping the iconic 'M' on Mario's hat and thereby making it a 'W' is a stroke of genius.
Flipping an 'L' upside down gives you nothing! You could conceivably rotate the 'L' 180° but that would only give you a '7', which doesn't really work when you're trying to name a character, but more importantly it doesn't work because conceptually rotating a letter is not as metaphorical as turning it upside down.
'Waluigi' as a name also doesn't even make sense. 'Mario' becomes 'Wario' by flipping the 'M'. However, because an upside down L isn't a character and isn't, therefore, pronounceable, any foil for Luigi cannot be called (upside-down L)uigi. So instead Camelot opted to use the 'Wa' designation, taken from Wario, and apply it to their twisted version of Luigi, presumably because they thought that prefacing 'Luigi' with 'Wa' was more indicative of the antithetical nature of the character. In algebraical terms their thinking might look like this,
If Wa + rio = - (Ma + rio)
then
Wa + Luigi = - Luigi
Which doesn't make sense.
So what we're left with is an upside down 'L' for a character whose name begins with a 'W'. Basically what I'm saying is that Waluigi is the BEST character ever created precisely because he's the WORST character ever created.