Mainly talking about in the FR setting, since it is the only official setting in D&D. If you want to make your gods weak to the point to justify a cleric/warlock multiclass in your world, that's on you. From a mechanical standpoint, I guess it can be good. For a paladin, it makes sense, not so much for a cleric, but from a story telling standpoint, I don't see why someone who already gets power from a,superior being will also try and make a deal from an inferior being to gain power also. In the long run, it actually weakens the cleric. As a DM, I would never allow any divine class multiclass into a warlock, unless you want to lose all your powers in that class, and you will have to seek your pact in the campaign.