In this context, 'fair' would mean 'sufficient but not ample' or 'adequate but not great' Definition 10 https://www.merriam-webster.com/dictionary/fair

School grades are indicated in a variety of ways. Sometimes they are "numeric grades" on a scale of 0 to 100, or 1 to 5. Sometimes they are "letter grades;" in the U.S., commonly A, B, C, D, F, with or without intermediate grades indicated by + or - signs. Sometimes they are just words, when the idea is to show that the grade is not very precise. One traditional set of words is: "Excellent, Very good, Good, Fair, Poor, Failing." As a grade or an evaluation, "Fair" means "acceptable, but not good." "Would you recommend XYZ as a house painter?" "He's only fair. No complaints, but I don't think we'll use him again."

BE FAIR TO = depending on context, it might mean something average, not especially good. The meaning of the sentence is that his marks were not excellent, they were almost disappointing, but at the same time, they were not very bad. Yes, you can use it all right.

No one likes to be called "fair" or "average". There is an expression, "damned by faint praise".