It probably wont hurt anything but just looks bad. It depends on if the guitar has a paint color finish or if it has clear lacquer. If it has paint finish solid color then you can sand inside the area of the dent enough so paint will stick to it well. You have find the exact matching color of the paint. After the sanding then use a good filler paste that will harden well and quickly within 24 hours or so. Then you sand carefully until the filler is even with the wood. Then you can paint over the filler and after the paint dries well you can very lightly sand with 400 grit sand paper preferably wet or dry sand paper.

Last thing is after about two weeks get a very light weight buffing compound and take a piece of cloth like from and old T shirt and hand rub the paint to buff it back to shine. Just be sure the paint matches the original so the color will not show as different after you buff it out.

If the guitar has a clear lacquer finish then you need to do the same process except don't use the wood filler. Get as much of the dent out as possible then fill in with clear lacquer. It may take several fills to get it level. Then do the same process to buff it out. If the dent is too deep you can use super glue as a clear filler. You can get a medium thickness from Stewmac.com. The nukber of the supper glue is 20X. Also use the accelerator they have for it. This will make it fill in fast and very hard. Then you can sand level then top coat it with clear lacquer.

Again go through the same buffing process and you got it. The dent wont really hurt anything on the guitar but I know it looks bad. Hope this helps you.