Gfrktyl

Would an aasimar's celestial guide whose connection to the aasimar only functions through dreams be a fair choice as a celestial patron for a warlock or would the connection being only dream-based be an issue?

Yes, an aasimar's angelic guide could easily be their celestial warlock patron, communicating with them through dreams

The warlock's description includes a brief exploration of the different ways in which a warlock might be in contact with their patron, including only via dreams (emphasis mine):

How does your patron communicate with you? If you have a familiar, it might occasionally speak with your patron’s voice. Some warlocks find messages from their patrons etched on trees, mingled among tea leaves, or adrift in the clouds — messages that only the warlock can see. Other warlocks converse with their patrons in dreams or waking visions, or deal only with intermediaries.

So the idea that the Warlock only converses with their patron via dreams is already given as a possibility by the Warlock's description.

And of the Aasimar's angelic guide, Volo's says that:

An aasimar, except for one who has turned to evil, has a link to an angelic being. That being - usually a deva - provides guidance to the aasimar, though this connection functions only in dreams.

You could read this as meaning that the link the Aasimar has to its guide only functions in dreams, but it's also a valid reading of this text that they can only communicate through dreams, and that they still have a mystical link to their guide normally. Practically, it seems like the limitation placed here is more about making sure the Aasimar can't actively petition their guide for aid during waking moments nor receive direct, explicit advice or information from them (since being able to directly ask a Deva or similarly powerful celestial being for information or assistance would be a bit excessive for a first-level character).

Even if the nature of an Aasmar's normal connection to their guide is more limited, there is nothing which would prevent the Aasimar and their guide from making a more significant pact such as that between warlock and patron - that's what taking levels in the warlock class represents, after all. Perhaps the young Aasimar was guided by the dreams sent to them by their guide to discover a holy site or relic, through which the guide was able to make more significant contact; perhaps this Aasimar and their guide just had an unusually strong connection to begin with! As a DM, I'd quite happily work with my player to combine the background of the class and race in this way. Thematically they work very well together and there's no compelling reason to disallow it.