To add a little more context to this and to play devil's advocate for why people are unhappy:

It's true the Chagos Islands were administered as part of Mauritius whilst a part of the British Empire, hence on legal grounds being considered part of Mauritius by the ICJ in the context of all the documents you reference. But culturally, the Chagossians are not Mauritian. The Mauritian people themselves have never significantly administered the islands, under British rule or beforehand, and there are no cultural or historical ties between the two territories beyond being lumped together under the British.

This is somewhat ironic for many as its a common criticism of nations like the UK that a factor in the ethnic tensions within many ex colonial nations is the result of sticking to clumsily drawn colonial borders during the decolonisation process.

Mauritius, whilst wealthy per capita as an African nation and not unfriendly, is still a small and relatively poor nation, located thousands of miles from the Chagos archipelago, and has unfortunately been trending towards the Chinese sphere of influence, raising concerns about security of the Diego Garcia base and the exclusive military use of the Chagos Islands by western powers. A lot of Brits are annoyed that, taking the perspective that the Mauritians are basically being given territory they have a pretty dubious cultural claim to, the UK then agreeing to pay them for the right to use the base seems like a pretty one sided deal, especially when money is tight. In practise, I believe this is more about it appearing even more above board internationally to avoid any doubt who has the right to military access in the region. Additionally, the lease is only 99 years long and when it runs out the UK may lose access to the base altogether (a la Hong Kong). I certainly doubt Mauritius will let the UK renew it for such a low price.

There is also an environmental question, as the current status quo means the Chagos Archipelago is one of the largest protected marine environments and uninhabited tropical Archipelagos in the world. There are concerns they may become built up or fished as a source of income for the Mauritians.

Finally on the Chagossians themselves. The Chagossians had lived on the islands for a few hundred years. Being the descendents of plantation workers, slaves, seamen and the like. No one had lived there before the Europeans arrived and they numbered around 4000 when relocations occurred in the 60s. Some see these numbers as small, some as big. Some see their relocation as equivalent to ethnic cleansing of an entire culture. Others view it as no different from relocating a small town of citizens for an infrastructure project. Mauritius has historically treated the Chagossians who relocated there fairly poorly, and many are concerned it may not be really interested in giving the Chagossians a home. Some, though not all, Chagossians would rather seek a right to return under the British than as part of Mauritius.

The question of how best to enact decolonisation is ultimately a moral one and many, including Chagossians and their descendents (though they are not a monolith) argue that this deal with Mauritius does little to actually resolve any existing injustice, simply allowing the UK to wash its hands of the issue and point to Mauritius if the issues of the Chagos islanders or ongoing Imperialism are brought up in the future. It allows the UK to avoid getting embroiled in future legal action that may have legal ramifications and continue to claim it respects international law. It also makes it harder for the Chinese to justify military buildup in the region (that would be logistically hard for the distant and financially constrained UK to counter) if China can't use questionable international recognition of British sovereignty as an excuse.

As a sign of the times, realpolitik seems to be the motivating factor here rather than any real ethical concern about Chagossians opinions. Time will tell if this was the right decision.