Is one sinful if he declines a wedding banquet invitation due to discrimination in the treatment of people, and showing off and hypocrisy, and is there any additional advice in this regard?

As long as you know that the walima (feast) was held for pride, ostentation, and reputation, this is a sufficient excuse to be absent from it, especially if it is compounded by the discomfort of differential treatment among attendees. Jurists have stipulated conditions for accepting an invitation to a walima, among which is that an invitation intended for a blameworthy purpose, such as showing off or ostentation, should not be accepted. It is disliked to eat food prepared for ostentation, and it is also a condition that the host should not specifically invite the wealthy because of their wealth.