Nothing ordinary about it!
That is, if the definition of "ordinary" that you are thinking of is "plain, dull, uninteresting, unexciting."
The root of the word "ordinary" as used in Ordinary Time is the one that means counted. As in ordinal numbers--remember those? I didn't either till I had to teach about them in first grade. Oh, I knew them all right: first, second, third, and so on. I just hadn't realized that there was a name for the category. (Don't you hate when that happens. There were a number of things I had to teach in first grade that I had never heard of till college.) BTW, the other numbers (one, two, three, and so on) are cardinal numbers.
So these Sundays are the weeks counted between the particular seasons surrounding specific feasts. They are all the weeks that are not part of Advent, the Christmas season, Lent, and the Easter season.
Are they ordinary in the other sense of the word? No. For each year during Ordinary Time we explore a different one of the Gospels in sequence--and what is more exciting than following through the life of Jesus and the extraordinary events that showed him as who he is--God made man.
Counted with ordinal numbers, but otherwise, not ordinary at all.
(BTW, we used to know these same Sundays as the Sundays after Epiphany and the Sundays after Pentecost. Some still do.)