If we're referring to mugwort as Artemisia absinthium, better known as wormwood, an ingredient in the true absinthe of days long gone, then simply throwing some foliage into a beer is probably haphazardous. At the very least, if it's approached on a gram-for-gram comapison to hops, then yes, you will likely have a bitter, undrinkable brew.
As with any plant material, while there are active chemicals/alkaloids that may be present, there's also a lot of undesirable chemicals that our bodies might not be able to process. Fortunately through evolutionary experience, the hop has been proven to present no ill effects to either mind or body when used as a beer additive, and pretty much everything that comes to the party is good for the tastebuds.
In saying that, such small quantities of wormwood in a brew aren't going to create any health issues, and a controlled quantity might actually bring unique flavours to your beer - but not used to the same values as the hop. Think of a comparison between one gram of household tea steeped into a cup of water, then try infusing a gram of hops for the same duration. I already know which one will have you running for the OJ when you take a sip.
If someone is adventurous enough to want to experiment with wormwood in their next brew, I'm happy to send 10 grams to the first member that PM's me with an expression of interest.