The problem as I see it is a that "show HN" is simply "here what do you think of this". If I - as a sort-of web-designer - look at the page and see a problem with alignments, lack of call to action, poor SEO optimisation or something then I want to offer that up; that's what I think.
Generally HN isn't the best forum to offer these more trivial thoughts - you really want to say "what do you think of this concept" or "what do you think about how I've implemented {concept X}".
Usually then if there's something that looks wrong but is not appropriate for discussion - like a spelling mistake - I'll take a glance to see if there's a simple contact form and use that instead. Emailing seems too much trouble unless I'm hoping you'll ask me to do some work for you(!).
Ugliness of the launch page seems quite important however and as it's subjective it's a point that could be discussed or on which the votes would help gauge the importance of the point.
If you feel the details are fine, or don't need work, then ignore the comments. Ditto for this comment.
When all you have is a launch page and no functionality demonstrating the features of the site, the points you mention are what many people are likely to focus on.
I did a show HN today for a restaurant analytics concept and people commented on the ugliness of the launch page and over pixels.