Is the exclusiveness of Christian denominations an issue for Mere Christians?

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Yes it is an issue; but then so is the opposite view an issue - with people believing that only their own micro-denomination is Christian.

Both views have their problems. 

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However, if we think of all real Christian denominations as being fundamentally not about beliefs (because beliefs tend always to conflict incommensurably. History shows examples where a very specific and high level, real or merely perceived  difference in a specific sub-doctrine can cause permanent schism as with the Coptic (supposed) Monophysites and the Eastern Orthodox church.  

But if we regard belief in a less micro-specific manner and instead think of denominations as different ways of practizing the Christian way

http://charltonteaching.blogspot.co.uk/2013/01/what-are-christian-denominations-ways.html

then the differences amount to no more than a recognition incompatible ways of life - if we live one when they we are not living another way - and this is a difference which does not really matter at the primary level.

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A real Christian denomination is a 'package' - and parts of the package may be absolutely necessary for the whole package to be a strong and sustaining way of life. 

Therefore denominations cannot usually be blended, and if certain elements are extracted from the package, then the package will collapse as a way of life. 

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For example, The Book of Common Prayer was, it turns out, necessary to the cohesion of the Anglican Communion - and when BCP text and usage was rewritten/ made multi-optional - the whole denomination became catastophically fissile and weak. 

Yet it would be silly to argue that all Christians (of all denominations) must embrace the BCP or else not be Christians. 

So, the BCP was necessary for Anglicans, but not for all Christians. 

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I think that's an example of how it works; and why on the one hand denominations ought-not to regard themselves relativistically but as having the truth (if not The Truth) - yet should not just tolerate other real Christian denominations but recognize them as fully Christian, but with different pros and cons for different individuals/ groups/ times and places.

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Note added: My intention  is not to deny that explicit affirmation and acknowledgment is unimportant, that would be an error, but to emphasize that belief means living-by; and therefore to understand the belief of another person - or another church or denomination - is not something than can be achieved merely by study of their explicit affirmation and acknowledgements.

In particular, this is not the case for the children, the inexpert, the simple in these churches - and yet for Christians it is clear that the greatest faith, the best of Christians, are to be found among these groups. Their understanding - and not the understanding of intellectuals, scholars, logicians -  is, in fact, the essence.

Thus, the simple understanding of the simple is the truth of Christianity. In the world, this is necessarily and rightly embedded and sustained by the church - which is complex, theological; and being a public body consists of affirmations and acknowledgments. 

Mere Christianity is to focus on the lived simple understanding of the simple as the unity of Christianity.