Was the mortality rate among Irish emigrants to Canada really ten times those to America? If so, why?

The ships that were used for the transport to Canada were called Coffin Ships. A genealogy page has this to say concerning these vessals

And so the first ships were commissioned and set sail, loaded with human cargo, for British North America (Canada). Many of these vessels were overloaded. Each held an average of 300 persons, some two or three times the number that would have been allowed by a port in the USA, and some were not seaworthy.

It also discusses a fare increase, causing higher prices for the ships arriving in the US

That following March, the minimum fare to New York rose to £7, an amount way beyond the majority of families facing starvation in Ireland. Even so, all tickets had been sold by the middle of April.

Considering that many of the fares were actually being paid by the Landownerws, price was important to them:

Parliament introduced new taxes (which landowners would have to pay) to raise money for 'public works relief'. The latter was a two-pronged scheme. It created work for labourers so that they earn enough to feed their families with food other than potatoes. And it provided for workhouses to be built to house the absolutely destitute.

The gentry were more than a little alarmed since they could see this taxation level, which they considered onerous, continuing for years. Some decided to bring a conclusion to their local problems by removing the burden altogether: by shipping their tenants to North America. They calculated that the cost of transporting each individual was considerably less than supporting that person in the workhouse for a year.

Another article, The Irish Emigration of 1847 and Its Canadian Consequences BY THE REV. JOHN A. GALLAGHER, C.SS.R., also discusses the landowners paying for passage:

By bearing the initial expense of transportation they would economize in the end. The victim was approached. An offer dazzling to one who was destitute was made. He was offered three Pounds by some landlords to pay his passage to Canada.

We also see the individuals that this offer were made to were often the weakest to begin with:

Another feature of this deportation shocks us -- it was selective. Mr. J. M. O'Leary, in his documented series of articles on Grosse Isle, says: "They took special care to rid their estates of the helpless widows and their little ones, of the old, the crippled, and those whose constitutions had been enfeebled by sickness and destitution."

Concerning the regulation on the number of passengers allowed in US ports, an act was passed in US congress in 1847, limiting the number of passengers to 2 per 5 tons of ship, with stiff penalties for overloading:

any greater number of passengers than the proportions aforesaid admit, with intent to carry the same to any foreign port or place, every such master shall be deemed guilty of a misdemeanor, and, upon conviction thereof before any circuit or district court of the United States aforesaid, shall,for each passenger taken on board beyond the above proportions, be fined in the sum of fifty dollars, and may also be imprisoned for any term not exceeding one year:

The ship could also be impounded and sold. So cheaper rates to Canada, and being able to transport more people per trip made the ratio of dangerously overloaded ships much higher on the Canadian route.

The overcrowded conditions also contributed to the typhus epidemic in 1847 in Canada, and caused the individuals to remain on their ships for even longer:

Dr. George Douglas, Grosse Isle's chief medical officer, recorded that by midsummer of 1847 the quarantine regulations in force were 'physically impossible' to carry out, making it necessary for the emigrants to stay on board their ships for many days.