Acording to bug template.

Bug overview description.
Long usernames make the comment hard to read. The reason is the username override the comment.

What part(s) of the site page(s) are affected?
Basicly, any page displaying user comments.

What is the expected behavior?
Restricting the username length or anything you consider correct.

What actual behavior is given instead?
Long username overrides the coment content.

Time of incident (if applicable).
N/A

Can you reproduce the bug every time?
N/A

What steps did you take to replicate this bug?
see any of this links:
https://e621.net/comment/show/3752188
https://e621.net/comment/show/3754026
https://e621.net/comment/show/3754388
https://e621.net/comment/show/3737628
(ok, this isn't that long enoght to make it hard to read, but mark a point)

Errors or other messages returned (if any).
N/A

Acording to feature request template.

Requested feature overview description.
Truncate the long usernames to prevent it to override comments.

Why would it be useful?
Prevent the username to override the comment, avoiding dificulties when trying to read it.

What part(s) of the site page(s) are affected?
Pages with comments. (general site, I supose)

My proposal.

Add this CSS code on anywhere you consider propper.

.comment > .author > h6 > a {
    text-overflow: ellipsis;
    overflow: hidden;
    display: inline-block;
    max-width: 100%;
}

This will truncate the username that exceed the width by adding an elipsis after it.