I'm not particularly familiar with that area per-se, but I once stayed in a hotel in Kamata (same reasons as you) that was in a neighborhood with lots of love hotels and the like; perfectly fine. "Seedy Neighborhoods" in Japan are not really what they are like, say, in the US, with a high risk of violent crime and the like.

If it offends you to walk by places that may be involved in less than legitimate dealings, then by all means look elsewhere, but it won't really be dangerous to a tourist who minds his/her own business.