So is it better to skip the differential port to avoid the floating voltages?
No, it all depends on how the model is wired in schematic. I use differential ports very often.
Can a SMD cap be considered a series element even when it is used as a shunt component where the second port is connected to a global ground as shown below?
Yes, this also is a single impedance where the input current into one pad is the same as the output current at the other pad. That can be modelled accurately by a single port between the pads.
But looking at your screenshot, I notice something unexpected.
Your Momentum ground is connected to schematic global ground, and still the ground voltage is not zero. It seems that your DC sources are not wired properly, and the ground potential of your source is floating, with no DC path to schematic global ground.
If all your Momentum ports are differential, and there is no connection of Momentum "ground" polygons to schematic global ground anywhere, results are not technically wrong but your reference potential is shifted. Zero volt is whatever
you choose as the reference potential, by connecting it to schematic global ground. So if there is no such ground path yet, you can connect one of the source grounds to schematic ground.
If that doesn't work, can you upload your ADS workspace or show the entire schematic?