looks fine to me – all quite readable, but it is your site so your call!
play with the following numbers
so overall you might try
/* Change margin */
@media only screen and (max-width: 499px) {
/* Change the width of the author column */
#bbpress-forums div.bbp-forum-author, #bbpress-forums div.bbp-topic-author, #bbpress-forums div.bbp-reply-author {
width: 50px;
}
/* position the left hand side of the content column */
#bbpress-forums div.bbp-reply-content {
margin-left: 70px;
}
/* hide the avatar */
a.bbp-author-avatar {
display: none;
}
}