Is the concept of God's sovereignty disputed outside of Reformed theology, and is God's sovereignty a label for God that is exclusively used in the context of Reformed theology?
While the idea of God's sovereignty is closely associated with Reformed theology, other traditions also deal with it. Here are a few examples from prominent authors from various traditions.
Lutheran: Mueller, Christian Dogmatics, 170:
Scripture describes God a) in His supreme majesty, as independent of anything outside Himself, or as absolutely sovereign in Himself, Rom. 11, 36 Viewed in this manner, God is not moved by anything but by Himself; or we may say, in Him cause and effect coincide.
Arminian: Miley, Systematic Theology, v1, 340:
God, who commands our prayer and promises the answer, is sovereign in the natural as in the spiritual realm.
But Miley argues that in Arminianism, "the principle of freedom" takes a central place, instead of sovereignty as in Calvinism (522).
Dispensational: Chafer, Major Bible Themes, 4, "The Sovereignty of God":
God is supreme over all. He yields to no power, authority, or glory. [...] When the history of the universe is completed, God's purpose and plan will have been wrought out according to His will even to the last degree.
Catholic: Ott, Fundamentals of Catholic Dogma, 183:
In the narrower liturgical sense one takes sacrifice to mean an external religious act, in which a gift perceptible to the senses is offered by an ordained servant of God in recognition of the absolute sovereignty and majesty of God.
Admittedly, Ott and Denzinger (The Sources of Catholic Dogma) seem to limit their use of the word sovereign, and often apply it when discussing the Catholic Church, not God.
Eastern Orthodox: Vladimir Lossky, in Ware, The Orthodox Way:
The Holy Spirit is the sovereign unction upon the Christ and upon all the Christians called to reign with him in the Age to come.
Search reveals no other uses of the term in Ware's book. I also checked Orthodox Dogmatic Theology, but didn't find this term applied to God.
Modern/liberal: Schleiermacher, The Christian Faith, 278:
In order to the realization of our personal continuance in union with him and, at the same time, of the perfection of the church, there is predicated an exercise of the sovereign power of Christ that puts and end to the propagation of the race and to the mingling of the good and bad, so that by one sudden leap the church, heretofore subject to a wavering growth, becomes perfect.
No other uses in this book, and none in Ritschl's Instruction in the Christian Religion.
The term sovereignty is of course extensively used in Reformed theology, but it's also used in other traditions. The authors cited above disagree on what God's sovereignty actually means, but for the most part they are willing to use the word.
That said, it seems to be used more in traditions more closely associated with Calvinism, whether because of shared history or because of historical debates – the Lutheran and Arminian authors cited above use the term more extensively than the others.