It doesn't particularly mean that that can't be trusted, but it does point to a poor level of customer service. It's something that would set alarm bells off in my own head, especially if they are a Dropshipper. Good communication is one of the most important things anyone should be looking for with a Dropship supplier because in the event of an issue with one of your customers, you need to be able to have fast and effective communication with your supplier to get issues sorted quickly to keep your customers happy.
If you don't have that communication, then it will more than likely be your reputation that suffers!
Now having said all of that, I should at least say that email isn't always the best way to communicate with a supplier. The old tried and true method of the telephone usually nets the quickest and most effective response. I know it's 2014 and suppliers should teat email inquiries as they do any other inquiry, but many still do not unfortunately. Many treat them as second class inquiries and only respond when they get around to it and many tend to ignore a lot because they consider that anyone serious will actually call them and anyone who emails is just wasting their time.
So my suggestion, give them a call and if the customer service on the phone is good, then thumbs up!