The plural seems better to me, because the sentence sounds similar to:
3. Girls should not wear a uniform, because it makes them look like a sack of potatoes.
The fact that "a uniform" and "a sack of potatoes" are used has to do with each of those girls not having to wear one uniform and looking like a sack of potates in case they do.
The determiner "no" before any singular noun will lead to the use of a singular verb but will call for the use of a plural pronoun if reference to that noun needs to be made, similar to what happens with "noone" or "nobody":
4. Noone/Nobody should wear a uniform, because it makes them look like a sack of potatoes.
Note: In all of the sentences above, I'd rather use "...because it will make them look..."