Is it possible to do something like this?
<input type="checkbox" ('@ViewData["Mailing"]'!= null ? @checked) />
Yes, it is possible:
<input type="checkbox" @(ViewData["Mailing"] != null ? @checked : "") />
but it is not something that you should want to do especially when you have strongly typed helpers at your disposal that already take care for this:
@Html.CheckBoxFor(x => x.Mailing)
Shorter, strongly typed, no ViewData, no spaghetti.