Since it is a french & english compound nickname, it looks like he didn’t manage to create “shithappens” so he made “merdehappens”, so you should consider it as if it were “shit”, since, I guess, people will read it as if it was “shithappens”…
Nevertheless, in french, the closer idiom to “shit happens” is “ça arrive d’avoir des merdes dans la vie”, which is, in my opinion, not very rude…
So it’s up to you.

About the same level as “damn it” (or is it “dammit?” whatever)

Side comment: In English “shit” is relatively higher on the vulgarity scale than “merde” is in French.

I don’t know if it’s really bad anymore. It is vulgar for sure, we don’t want kids to say it, we don’t use it in a meeting (usually), or on TV news, but it is really mild IMHO by today’s standards.

… in a mixed-ages (13+) community

I think it’s fine within your peer group: I guess that almost anyone except the most polite could use it occasionally when speaking informally with their peers. I consider it “colourful” language rather than “offensive”.

It’s less OK across/between social groups: for example I expect a teacher would avoid saying it in front of or to their children, and vice versa.

As such I feel it’s slightly exclusionary: use of that name signals to me that either I must be part of their social group, or that they don’t care about (are ignoring) my presence, are talking as if I weren’t there.