No it isn't, if Lukaku isn't good enough to be the main penalty taker that is his problem. Anyway, you make it sound like all Kane does is score penalties.
Let's look at the stats
Kane 22 apps, 19 goals.
Lukaku 26 apps, 18 goals.
Even taking away Kane's 4 penalty goals, at his goals per game rate this season he would still score more than Lukaku.
Kane has 68 goals in 108 PL games. That smashes Lukaku's goal record by a distance.
This stat actually proves the opposite quite nicely when you factor in that Spurs are a far superior team with far superior 'suppliers' and goalscorers and Lukaku is the more creative player.
Kane would be further ahead if he was as good as Lukaku. Lukaku wasn't far off him last season either despite playing in an utterly atrocious Martinez side while Spurs enjoyed a title challenge. We were just as bad the year before while Spurs were top 6.
It's pretty clear to me that Lukaku is a better goalscorer and more creative and therefore the superior player. Kane'll have to outscore Lukaku by more than 1 with pens to make up for the yawning gap between the two in terms of the service they get.