In order to preview the video in your uiaxes, you can create an image and preview the video on that image:
% Image acquisition
vid = videoinput('winvideo',1);
% Create blank image
hImage = image(app.UIAxes,zeros(720,1280,3)); %If image resolution is 1280x720
pause(2)
% These lines set proper aspect ratio
app.UIAxes.XLim = [0,1280];
app.UIAxes.YLim = [0,720];
app.UIAxes.XTick = [];
app.UIAxes.YTick = [];
pbaspect(app.UIAxes,[1280,720,1])
% Preview the image
preview(vid,hImage)
