I think what you have is fine, except I would strike "is"; I would try to eliminate one use of "target"; and I would use a different word for "single", like "specific" or "individual":
The more varied the offer, the harder it is to reach a specific target group in your marketing.
That said, while you can say that an offer is "varied", you typically don't call it a "varied offer". You might call it a "miscellaneous offer," but "miscellaneous" is really describing the offer, not the things being offered. You typically use a different construct with words like "various" and "variety".
I don't really have any sources to back up my claims but I can provide a few more examples:
My reasons for wanting to leave are varied.
I have various reasons for wanting to leave.
I have a variety of reasons for wanting to leave.
I have varied reasons for wanting to leave. <-- LESS COMMON
The sizes and colors of the clothes are varied.
The clothes come in various sizes and colors.
The clothes come in a variety of sizes and colors.
The clothes come in varied sizes and colors. <-- LESS COMMON