Short answers are: yes and yes (if smoke ring is important to you).
Long answer (why and how smoke ring forms): smoke ring will not form on fat, and fat does greatly reduce the reaction of smoke chamber gases with the meat underneath a fat-cap. The thicker the fat the less likely you will find a smoke ring under it. Notice in this photo of a beef brisket point I smoked several years ago with lightly trimmed and deep cross-hatch scored fat-cap that the smoke ring is almost non-existent under the fat cap, while it is much more prominent on the lean portions of the surface...same brisket point muscle, slices placed as cut from the muscle (I don't have many photos of good examples with pork loin, but these explain it best with photos I do have):
With a thinner fat-cap, there is some formation of smoke ring, but it's formation is reduce mainly to just close proximity to cross-hatch scoring of the fat-cap, which translated to less distance to reach the meat, or less density of fat for smoke chamber gases to pass through, however you want to look at it:
And, a few prime examples of what lean trimmed brisket point can yield for smoke ring formation...notice the tubular muscle construction with fat throughout, and the lack of smoke ring in the fat, yet, the meat is screaming with smoke ring:
Bear in mind that smoke chamber temperature, meat surface temperature (lower is better for smoke to stick on the surface), humidity (higher is better for smoke to stick), smoke density and chamber ventilation all play a role in how much smoke particles stick to the surface of the meat, and this has a direct relation to flavor. I should inform you that, contrary to popular belief, smoke ring is not formed from the smoke itself, but rather from the reaction of Nitrogen Dioxide (NO2) with the meat, which forms nitric oxide, creating a temperature stable pigment in the meat. Colors range from a deep red in beef, to a light red/pink in pork, and a even lighter pink in poultry (at times being so light it is difficult to detect without close observation). Smoke ring typically can not be achieved in an electric smoker without the use of a fuel as a secondary heat source because NO2 forms from combustion of some type of fuel in air, containing nitrogen gas (charcoal, lump, wood, LPG, natural gas). NO2 forms at combustion temperatures of approximately 600*F and above, which is generally higher than temperatures at which smoke woods will smolder and produce smoke.
That said, smoke ring has no effect on flavor, but is still considered by many to be one sign of good BBQ...along with overall texture, bark, moisture, tenderness, etc. And, hey, who doesn't revel at the sight of a deep and prominent smoke ring, huh? It is one of my goals, because when I see it, I know I did well...had good temps for smoke to stick to the meat for longer periods of time...which in turn can also translate to better natural tenderizing of a tougher cut of meat by cooking slower and allowing the collagen (muscle connective tissues) to melt into the meat.
So, with this quest for smoke ring, yes you can trim the surface fat away if you wish. Some don't for fear it will cause meat to become drier when finished and they feel the fat-cap self-bastes the meat and helps retain moisture as it renders down during cooking...I have mixed opinions on that, and depending on the interior fat content, I may opt for trimming or complete removal of the fat-cap. If the muscle is well marbled and/or has fat layers between multiple muscles it will take care of itself during cooking. I have successfully used a method for smoking lean meats with lean trimming as well. If cooked to proper internal temps, few meats will dry out, unless there are improper cooking methods in use, and, temps get away on you at least somewhat. I have trimmed the fat-cap off many cuts of beef and pork and been satisfied.
I say trim away, and enjoy the benefits...with pork loin, minimum internal temps of 145*F is considered safe for USDA inspected meats...I go bit higher, into the lower to mid 150's range after carry-over during resting...never had a complaint from anyone.
I gave you a big bite to chew on...just thought you'd like to know. Smoke on!!!
Oh, you may find this rather interesting and informative...research, trials of theory, and documented quite well by a another member who I have a great deal of respect for:
Understanding Smoke Management
Multiple edits: typos while dealing with touchy mouse pad...sorry.
Edited by forluvofsmoke - 7/3/15 at 11:58pm