I would feel a little oogy about having just a diaper on my toddler and nothing over it. Not because it's immodest, but because if she's scooting around on her rear (I know the dress was long, but my kids at least had an uncanny inclination to hitch any length of dress up around their armpits while playing), the diaper can fuzz up and possibly even leak. To prevent this, I put little shorts on under my girls' dresses and skirts, even if they didn't have the specific bloomers that matched the dress.
And EllenS, I'm thinking she meant the bloomers that often come with little dresses, not a cloth diaper cover. The bloomers are often called diaper covers, too.