I think there's two things to consider here.
1. Are you unhappy because you're not proud of what you're working on?
2. Would you be happier elsewhere?
As someone who's done a lot of work for a lot of clients over the years, you learn eventually that even in the best jobs, it's not all rainbows and unicorns all the time. It's about the ratio of good to bad.
Now, on the one hand, you said this app is all HTML and it shows. Is this a concern you've raised, and if so, what was the response?
Secondly, from the standpoint of someone who's run a business, whilst my default position is strive for awesome, when you've got budgets and employee time and client billing and other such things to worry about, sometimes you have to decide what's acceptably good, and anything past that is great.
From a businessman's point of view, the measure of success isn't design quality or usability or anything else, those are just tools to the main goal: profitability.
So, if you haven't had the concerns discussion with those higher up the chain yet, bare those things in mind. Understand that your concerns and theirs will be different. This will result in them taking your concerns more seriously, as it'll show that you're not just concerned with wide-eyed idealism, but with wanting to help further their goals, by making the end product better, thus delivering more profit. Sell the business case for your viewpoint.
Failing that, if you do leave, be realistic about your prospects for a new job. You're not going to be doing high-end super-cool exciting thing yet, but that's OK, because by not doing those things, you can learn how what you do fits in to businesses. That's valuable, and will make you a much more capable developer long term, because you'll be able to relate to the client, the boss, and your work together. That's killer.
Regarding the product itself, I've found that there's ways to find even the most boring things interesting. Just gear it around the things you do find interesting. Nothing, but nothing, is just flat boring. Which isn't to say some things are unpleasant, as they are. But not boring. There's always ways to make work fun.
Hope this helps. If there's anything further I can do, reply here or drop me an email - I'd be happy to help.