The simple fact is that we don't really know the actual incidence of rape at any point in time, even arguably today - between the need for it to be reported and the many barriers that have stood between victims of sexual assault and reporting it to local law-keepers (let alone a prosecution, especially let alone a successful prosecution), any statistics we might be able to point to from the courts would be woefully inadequate. Only a few cases for rape came to the High Court of Justiciary in Scotland over the course of the entire century, let alone the 1740s.

That being said ... there was a high incidence of rape in eighteenth century Britain in general. The term "rape culture" absolutely applies here - sexual assault was incredibly normalized as a part of life, with popular/folk ballads treating it as an effective means of gaining a woman's love. For instance, the broadside ballad "In Heriot's-Walks" (1715) (you can read the full text here) begins with the "hero" meeting the woman he's in love with in Heriots Wood; he tells her that he wants her and she tells him to cool it, so he says he would never, ever rape her, which leads her to allow him "a modest, Chast, and Civil kiss". As he comes forward, she falls down backward (he assumes on purpose) and he takes the opportunity:

When she was down, what Mortal Creature

Cou'd have refrain'd the yeilding Dame?

Thus Opportunity and Nature,

Reason and Honour Overcame,

I gain'd my Point, through Love's Assistance,

She blusht for my not proving True;

Not for her own kind Non-resistance,

What could a poor Weak Woman do?

She says she's angry, calls him rude, and threatens to scratch him, but he continues and the end of the song gives her perspective:

I did not think you'd prove so Cruel,

As all my quiet to destroy,

Read in my eyes my grief, my Jewel,

But in my Heart my coming Joy.

The ballad is ambiguous at many points as to whether the man is only telling himself that she secretly wants this assault or whether the woman is actually flirting with him, but the ending rather explicitly states that she is genuinely hurt ... but also that the assault (or perhaps more specifically, her climax) is making her fall in love with him.

Another, "The Coalier Lassie" (1701) (original here) does not even have the excuse of depicting a single event in a somewhat ambiguous relationship, but takes the form of instructions:

Although at first she may deny,

yet you must pursue her:

Her Modesty will make her cry

your Rudeness will undo her.

But with a forward brisk Address

unto the point intended ;

First you must Kiss & then Caress,

and so the Matter's ended.

Each verse of the song is quite explicit: if you want to have sex with the bonny collier's daughter, you should just assault her and ignore her struggles and cries, because once you're finished she'll love you and happily go with you next time.

There was no clear distinction in the popular understanding between seduction and rape, with assaults committed by men who were wooing or courting a woman particularly tending to be classified as "seduction", with the strong implication that the woman was greatly to blame for giving in. In Scotland, although the High Court legally was the only one with jurisdiction to hear rape cases, in practice sexual assault was very frequently handled in kirk sessions as sins committed by both parties, like fornication. The many accounts from women who used their sexual assaults as evidence of marriage bear this out, as well as the normalization of rape itself. While it was neither legally binding nor sanctioned by the church, a declaration of marriage followed by consummation had been an informal means of legitimating a relationship among the non-elite for centuries, and so many women who were able to get a promise out of their attacker before being raped used the event to make a declarator-of-marriage case in court. A 1782 case featured a maidservant asking the court to uphold or enforce the marriage between herself and her employer after he had locked her in a room and violently assaulted her, then promised marriage, after which she allowed him "every privilege of a husband" on later occasions because she understood their relationship to have changed. (The court did not uphold the marriage, most likely because of the class disparity.)

Sussing out exactly how sexual assault affected its victims in the period is difficult, beyond the obvious fact that violence is traumatic, particularly violence as intimate as sexual assault. Many women would respond as we would expect: eighteenth-century Scottish writer Jean Marishall argued that a woman who married her "seducer" would live the rest of her life in fear, and some prosecuted their rapists despite the fact that publicizing their "ruin" would result in their remaining single or even having to turn to sex work. The many women who did continue to engage with and even marry their rapists must be regarded in context with the fact that even while emotional, affectionate relationships became more expected between husbands and wives over the course of the century, marital rape continued to be completely unthinkable as a concept: in such a society, how can pre-marital rape really be comprehended? How is it conceptualized? Just as we can't really know the extent of the existence of PTSD in pre-modern soldiers, we can't see into the psyches of women who had been conditioned to believe that rape could be solved with marriage, or that it was an acceptable step in courtship and a form of love. We can't know exactly how they aligned the trauma they faced and testified to with their decisions to remain close to their rapists, except to know that they were forced to.

I have here been largely discussing the situation for Scottish(/British) women throughout the century, but I should discuss the period of the Jacobite Rising more specifically, because it's important context. While I've made it clear above (I hope) that sexual assault was highly normalized in British culture when the assailant was known to and even close to the victim, rape could still seen as a form of violence and stigmatized. Scottish Highlanders were seen as completely uncivilized "savages", and tales that they were rapists themselves were used by their opponents to justify harsh military reprisals: they supposedly were beyond the reach of diplomacy and similar means. In 1745, John Dalrymple, the Earl of Stair and a high-ranking military officer, had suggested that rape could by used as a weapon against the wives and daughters of those Highlanders fighting against the Crown to make the men return home to defend their women, and that soldiers should simply be sent to "live at discretion" in the Highlands, where they might make violent nuisances of themselves and lay waste to farms and the countryside. After the Battle of Prestonpans in September of that year, the "live at discretion" tactic began to be adopted by commanders in the Highlands, and by the following spring it was official policy; while the soldiers were not directly encouraged to assault the women they came across, the already-existing rape culture in British society made it seem a reasonable option, particularly when crossed with beliefs about the worthlessness of Highlander lives, and they were certainly not punished in any way for it by their officers, who typically agreed with Dalrymple's suggestion. In the specific context of Outlander's setting, yes, rape by British soldiers against Highland women was extremely common and was deliberately done as a form of violence to terrorize them and their families.

Sources:

Kate Barclay, "From Rape to Marriage: Questions of Consent in Eighteenth-Century Britain", from Interpreting Sexual Violence, 1660–1800 (Routledge, 2015)

Deborah Simonton, Women in Eighteenth-Century Scotland: Intimate, Intellectual and Public Lives (Routledge, 2016)

Geoffrey Plank, Rebellion and Savagery: The Jacobite Rising of 1745 and the British Empire (University of Pennsylvania Press, 2015)