The first siman in the Mishna Berura (MB) chelek 3 is about how to maintain the honour of Shabbat:
דהיינו לקדשו ולכבדו בכסות נקיה ולענגו בעונג אכילה ושתיה
...To sanctify and honour [Shabbat] with clean clothes, and to take pleasure [in Shabbat] with [good] food and drink.
Within this siman, the Shulchan Aruch recommends the practice to launder ones clothes on Thursday in preparation for Shabbat.
Are there any sources that specifically mention/recommend wearing formal/smart clothes, other than just 'clean clothes' (as mentioned above)?
The enjoyment of food can be considered subjective (see here) despite the MB recommending (amongst other things) fish, meat and wine. In order to enhance oneg one can choose foods which they enjoy eating.
Can the same subjectivity be said by clothes on Shabbat? If you enjoy wearing sandals, jeans and t-shirt and despise wearing a suit/tie due to its lack of comfort (or where adverse weather conditions make it difficult), is this ideal? Or do the 'kiddush', 'kibbud' and 'oneg' mentioned in the MB have much more specific applications?