I would say that's an inherent flaw of referenda, and why they should be generally avoided, rather than something undemocratic per se. I guess a referendum can exploit the weakness of democracy though.
How is it not democracy? We voted, the government and Parliament acted on that vote, people have protested, MPs have resigned and we are were we are. MPs will vote again and we will either get May's Brexit deal, a slightly modified deal, a significantly different Brexit, no Brexit or potentially...
Gone for May doesn’t get plan through, leadership contest or General Election – another referendum, however I wouldn't be surprised if its any of the others apart from 'no deal', no one wants this.