Six-month suspension? I would certainly agree with you, had the umpire been the target of his outburst. However, he was not, so let's stay reasonable. Yes, he lost his cool, no question about it. But I would think that Mengel is not really that stupid. He probably knew right in the moment he was kicking the towel box, that he would get into a little trouble. I don't know how the TTBL handles stuff like that usually. One or two games suspension and a fine, and that should be it (from my point of view). He was not the first and he won't be the last (unfortunately) to kick a towel box or the table...
More importantly however: Quite a few kids are present during these matches (as seen in the clip) and since these Bundesliga players (or pro players in general) are often seen as role models by these kids, situations like these should be communicated accordingly within the clubs (and in that case particularly within Mengel's club). Behaviour like this will not get you anywhere, aside from getting a suspension and a fine and making a fool of yourself (at least for a certain period of time). In fact, I would hate to see an increasing number of ten, twelve year old kids kicking tables, towel boxes and maybe people, thinking it might be cool or thinking they will get away with it since other people were getting away with that, too. It is of course the duty of the parents and the coaches to teach them that.
But as I said earlier: He kicked the towel box, he did not kick or kill the umpire, so let's stay reasonable and let it be (more or less)...