Hola Laura 

Your sentence would have been correct as well as:

"lo menos atractivo de él es su físico"

However, the structure of the sentence is different here. We put a note so you can see the literal translation. It's saying:

_____ menos atrae de él es su físico. 

 

(The least attractive thing about him is his appearance. 

[lit: The thing that least attracts about him...])

 

So, here, you can't use just "lo" because that would be saying:

"lo menos atrae de él es su físico"

This wouldn´t be grammatically correct. You need "lo que menos atrae..." (What attracts the least..." 

For "lo" to be correct you need an adjective:

"Lo menos atractivo de él es su físico."

This means pretty much the same, doing a superlative. See this lesson here; at the end we use examples with lo + más/menos + adjective.

I hope it helps clarify it.

Saludos

Inma