Is it considered hypocrisy to beautify one's prayer when people are present and shorten it when one is alone, and do good deeds become nullified because of it?

Performing salah (prayer) well in the presence of people but neglecting it when alone may indicate ostentation (riya') and corrupt intention, which is a form of minor shirk (polytheism). Performing many supererogatory prayers (nawafil) in the presence of people but abandoning them when alone may indicate a lack of sincerity. A Muslim must be mindful of Allah in their worship, not seeking the praise of people. As for rejoicing in people's praise without the intention of ostentation, we hope there is no harm in it, and it may be glad tidings for the believer. The advice is to strive for sincerity to Allah, to maintain prayer when alone, and to perform many sunan (supererogatory acts) at home, because people can avail nothing against Allah.