It is unlikely that the SMPS is what is causing your issues. This is even what you're observing: the touch screen works fine if the SMPS is powering it as long as there is a protective earth ground connection.
Therefore, the SMPS is not the problem. If it was, the issue would be there regardless of any earth ground connection.
The touch screen and everything else in the device is still grounded to the SMPS, any noise that is due to the SMPS is still there with or without a protective earth ground. So attempts to filter out noise that is not actually causing a problem in the first place is not going to be particularly helpful.
The real issue is that capacitive touch screens lose a lot of sensitivity when they are isolated from earth ground, as this creates a parasitic capacitance between the panel and earth ground/everything else. In effect, this acts like a capacitance that is always in parallel with the screen.
As far as the touch screen is concerned, humans and their fingers are little more than meat-based capacitors, and this capacitance acts as a secondary path for a tiny AC signal (~200kHz typically), and is diverted through the meat capacitor when the screen is touched.
If there is a small capacitance in parallel with this meat capacitance, this results in there being less change in the signal when meat capacitance (a touch) is added. The capacitances involved are both in the range of single digit picofarads, so the presence of that extra screen-to-earth ground capacitance can have a large impact on the change in the signal seen when a user touches the screen. Thus, sensitivity is reduced.
There are ways to help mitigate this. If the panel has a metal frame, bonding that to your device's ground via a screw and wire lug can increase sensitivity a little. However, this is typically something that you would fix using the touch screen controller itself. Most controllers have registers that allow you to adjust sensitivity thresholds for exactly these situations, or have a 'training mode' where one holds a finger on the touch screen for some number of seconds while it calibrates the sensitivity.
I can't really provide more information than that as you have not provided any information about the touch screen or the controller. I would suggest looking up the part number for the panel and/or the touch screen controller and finding the data sheet or other documentation. It should have the information you need.
