Learn German: Die Hitze hat mir die Glasur ruiniert. - The heat ruined the icing for me.

Using the dative mir (for me) instead of the possessive meine (my) emphasizes that you are the victim of the event. This is the dative of disadvantage. It makes the sentence feel more personal, showing that the heat did this to you, rather than just stating a dry fact about your icing.

Why is mir placed before die Glasur?

In standard German word order, pronoun objects like the dative mir almost always come before noun objects like the accusative die Glasur. If both were pronouns, the accusative would come first, as in: Die Hitze hat sie mir ruiniert.

Does die Glasur always mean icing, or can it refer to other things?

Glasur generally means a glaze or coating. In baking contexts like this one, it translates to icing or frosting. However, you will also see it used in pottery to refer to the ceramic glaze on a pot or tile.

Could I say kaputt gemacht instead of ruiniert in this sentence?

Yes, Die Hitze hat mir die Glasur kaputt gemacht is perfectly natural and means the same thing. Ruiniert sounds slightly more dramatic and is a great loanword to use, but both are very common in spoken German when complaining about a mishap.