Photoshop Touch wasn't designed specifically for what you describe but could be used. There are some limitations and considerations as the workflow would be a little different than what you described. I did something similar to what you described and will share what I found. Also note, Photoshop Touch is still being improved, so what I describe here may not always be this way.
I created my talk bubble in Photoshop CS5 using one of the native custom shape tool presets. I just created a large one with a stroke around the edge on a transparent background and saved it as a PSD, which I then uploaded to my Creative Cloud account. PS Touch is a raster rather than vector based application, so its not possible to import vector shapes (.AI files) or work with image data in a vector format. Text is vector when being created but raster once created. Overall, I would say that the workflow is much more labor intensive than doing something similar in Photoshop, just because this is a simpler less complex application so consequently requires many more manual steps.
Another significant consideration is what you are doing with images after completion, are they being printed, posted online, etc. PS Touch currently supports a max image resolution of 1600x1600, which is fine for images being posted online, but might be too low for some printing in some cases. The last thing that I discovered that I would want you to know relates to working with text, the software does include Cooper Black, and comes with about 30 different font styles (limited to these as they are built into the app itself). When creating text, one can only create a single line at a time (no return to make two lines) and it is not possible to define a set size (no point size option) so you would have to scale them manually to match. The last point relates back to vector, once a text layer is created it cannot be edited, so if there was a typo or you wanted to edit the text you would need to recreate it.
Photoshop Touch is probably one of the most complex image editing apps available on the tablet today but it’s not as powerful as Photoshop on a computer. This is currently not possible with tablet hardware and software capabilities. I have seen apps in the App store specifically designed for creating talk bubbles and text on images, I haven't used them but I know they exist. If it were me I would probably explore all my options (software on tablets is generally much less expensive than desktops anyway). Even if Photoshop Touch wasn't the perfect match for this exact workflow it might be worth the investment just to have it in your arsenal of tools for other editing or image alterations.
Hope this helps,
Okay, many thanks!