If going to church is anything like my synogogue, I would think that you should be dressed appropriately. After all, I do believe that you are going there to communite with G-d. Hopefully you should be dressing with that in mind.
At my synogogue (which is orthodox), there is a focus on modesty. Skirts should be below the knee and shirts need to go past the elbow. Of course, these are minimums. Basically, when you are praying the focus should be on prayer, not clothing. In my opinion, it is best to dress quietly, modestly and with respect.
If someone does want to dress differently though, this is of course their perogative. I have seen some girls coming in with sweat pants and sloppy tops. Although I do not personally like this, I think that it is positive that they are attending at all. At the same, I do think that one should dress according to the situation. If I was visiting a church, for example, I would still want to honour their customs and habits of dress.