Ill just copy paste a response I gave to a similar question to this with some minor revisions. It's worth noting though that most war in the American Revolutionary War were "standard" battles. The most "guerrilla" fighting happened on the relatively smaller Georgian front.
Skirmishing tactics were by no means considered "cheating", "dishonorable", or "cowardly". We can look at the American War specifically but it will be more helpful to speak more generally for all of Europe in terms of skirmishing tactics. In 1964 and in 1976 Peter Paret found "little evidence that they [Europeans] were shaped in any essential way by colonial experience. . .European armies acquired very little that was new to them from the American War." However we will be using for our principle source Johann Ewald.
Johann Ewald (1744-1813) was a Hessian (German) military officer and wrote two great treatise on contemporary warfare. While we can't quite use it as a perfect history it serves as a sight into the eyes of a contemporary who saw the American Revolutionary War with his own eyes (German mercenaries were quite common as auxiliaries to the British) and served with the Danes writing his primetime piece Treatise on Partisan Warfare. Basically it is argued (and is commonly held in thought to this day) that by very virtue of an absolutist Europe armies were strictly regimented and drilled. Ewald would say
*"The permanent military establishment proved an expensive but indispensable component of absolutist rule. In domestic policies it served as a means to bind the nobility to the state. . .In foreign policy it formed a readily available instrument for power politics. . .It's ranked and file, however, lacked a feeling of loyalty to the state or dynasty. Levied from among the lowest social classes, forced into military service. . .the standing armies of Europe. . .were held together by brutal force and blind drill. Too great of an investment to be risked lightly in battle and liable to desert at any moment, they were most efficient in a pitched battle over open ground, where superior drill, volume of fire, and sheer numbers decided the outcome of battles. Any kind of individual action was almost impossible with such. . .troops."*^1
So in some sense, yes, musket based warfare was born with an abhorrence to skirmishing based tactics. However it was not out of 'cowardice', 'honor', or 'fair warfare' but out of necessity. The standing army provided a means to force the nobility to share fate and allegiance with 'the state' but also created a system where the 'rank and file' had to be brutally suppressed and drilled and could not be allowed to just let off on their own. Skirmishing tactics require highly trained, intelligent, and most of all free willing soldiers.
So the very basis of why absolutist warfare existed along with its application (at least in Western/Central Europe) did not allow an irregular force to develop independently. This is also because "Little War" as it was commonly called required highly trained and motivated officers as it was a 'continuous war' and was a strictly tactical field. It was not an attractive field. *"Continually in action yet only supportive in function, outside the support structures and promotion criteria, unconventional in actions and dissolved at the conclusion of peace"*^2 -- noble officers did not want any part of this since it held no glory or advancement like the 'regular' army until much later.
More importantly though it required disciplined, quick thinking, self motivating men rather than rank and file blindly drilled and only operating out of fear of punishment and by the very basis of war described above, before popular participation and nationalist thought, irregular/skirmisher based warfare would be a difficult area to find recruits for that reason. This would come easily though first for the Austrians with their fiercely loyal Croats and Hungarians who were happy to, for instance, act as Grenzers (commonly referred to as 'peasant soldiers') in the military cordon against the Ottoman border which basically operated as a demilitarized zone on the Ottoman border where it was only comprised of these 'peasant soldiers' acting as a first line of defense.
The very question you asked is answered quickly though; were Europeans seeing this type of warfare as 'new' or 'cowardly'? Certainly not. We know from for instance Marechal de Saxe in 1732 that this isn't true as he provided one of the earliest definitions of irregular warfare as being waged by describing it as
*"small detachments, consisting in patrols, raids, the gathering of information, ambushes, reconnaissance, protection of the main army from surprise attacks, foraging, capturing of prisoners, and the like."*^3
This is becuase, Ewald would argue (though I would disagree slightly), the rigid nature of absolutist warfare required the 'practical' solution of irregular warfare to augment it eventually. Werner Halweg said:
*"Regular war, which is determined very much by learned theories and systems, gets assistance from irregular war, which itself is determined by practical needs, and only the mutually beneficial cooperation of both forms creates the reality of war in the eighteenth century."*^4
This necessary practicality first would climb out of the primordial ooze, as explained prior, from approximately 45,000 hunters of Eastern Europe in the mid 18th century who absolutely thrashed the Prussians in the Silesian Wars at first. So the disciplined, self motivating, quick thinking troops came from the regular old hunters in the 'fields' of Eastern Europe -- very similar to the American mythology of the regular country men rising up against the line but only decades prior. In this case though it was performed by Croat and Hungarian irregulars who were fiercely loyal by default to the Habsburg crown. In response to this the Prussians, despite having Frederick the Great at the helm who abhorred their use, were required to use them in response just to survive.
Your second question of was it controversial? You bet your ass it was but not at the time of the American Revolution; by then it was an accepted necessary part of warfare (as briefly mentioned earlier with the Croats thrashing of Central Europe with 45,000 of them).
TL;DR:
Was the use of irregular forces controversial? Yes.
Was it new or in some way avoided out of 'honor'? No, it was seen as a necessary, albeit brutal, part of what 18th century warfare was becoming.
Notes
^[1] Johann Ewald, Treatise on Partisan Warfare, 9
^[2] Ewald, Partisan Warfare, 11
^[3] Carl von Clausewitz, Varlesungen uber den Kleinen Krieg 446
^[4] Quoted from Johann Ewald, Diary of the American War: A Hessian Journal
^[5] A Treatise upon the Duties of Light Troops, no page provided.
Further Reading:
Johann Ewald, Treatise on Partisan Warfare