Full discloser - I've never been to a convention, dressed in cosplay, or even ever seen anyone in person dressed in cosplay. My only knowledge of the social activity is from features and articles I've read on Anime News Network.

However, from what I've seen online anyone can dress up as any character they want. Men as female characters, women as male characters, thinner people as muscular characters, etc... you get the idea. What probably matters most is the amount detail and effort people put into their outfits, the more detailed the outfit the more easily recognizable the character they're dressing up as is. In the end though what ultimately maters is that you're happy and comfortable with the results and are having fun regardless of any opinions or comments you may get from anyone else.

As for cosplaying as Hestia specifically I'd say she's probably one of the easier characters to dress as. To be recognizable you probably only need a few things - your hair or a wig tied up in pig tails with blue and white ribbon, a white sleeveless mini dress maybe cut to look a little ragged around the collar and shoulders, a blue ribbon tied in a bow around the neck, white gloves, and another blue ribbon tied around the arms and chest. That last item is obviously key, anyone who sees that will probably know who you're cosplaying as regardless of any other concern you may have, although I'm not sure how it's supposed to stay in place. (Maybe sewn to the dress and stuck to the arms with double sided tape?) Either way the outfit seems like it should be one of the simpler and cheaper ones to make assuming that's a concern.