While many graffiti designs are nothing more than the artist's graffiti pseudonym and do not seem especially creative, there are undeniably some artists who show imagination and talent.

I know the litteral meaning of the underlined part, but what does it imply? Thanks.
Angliholic there are undeniably some artists who show imagination and talent.

The qouted sentence helps you to understand this, I guess.
Thanks, Doll.
But I still don't get the underlined part.
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You probably know what graffiti are: they can either be drawings drawn on walls, or words written on them.

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The first part of the sentence refers to graffiti similar to the ones in the above pictures and the whole sentence discusses whether drawings like these one can be regarded as a form of arts.

I think your sentence means something like "Some graffiti represent little more than the authors' pseudonym (sort of a nickname) and they are not particularly artistic and original, while others do show imagination and talent."
Why do graffiti on a wall Emotion: smileBody art
In here where I live, I believe people refer to artists' graffiti pseudonyms as tags. 

Talking about artists who show imagination and talent, check out these ones by the mysterious 'Banksy'. Emotion: smile

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which underlined part?