Actually, the default piano is an emulation of an "acoustic" piano.  The quality of this emulation will depend in part on the quality of your sound card and its on-board synth engine.

In MM-15 Vita, there are several acoustic piano's to choose from, some of which may be more pleasing to your ears.  The Acoustic Pro 1 and Acoustic Pro 2 may be more of what you are looking for.

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You can click on the down-pointing arrow to the right of the track number and choose a different VST instrument from the drop-down menu.

You can also double-click on the object in the arranger to bring up the MIDI editor.  You can choose a different instrument from the menu at the top of the screen.

You can also use the Instrument Editor to modify the piano sounds.  You can access this through the Object menu.  The piano's do not respond to the envelope and filters, but you can alter their sound using the dynamics adjustments as well as the reverb and delay settings.  The reverb and delay must be turned ON to have any effect.

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