Sniffles and Lumpy can vary between like and hate in the series. While they can be friends at some points, there are times in which the two share a conflict, mostly due to their opposite IQ scores.
The majority of the time, Sniffles gets along with Lumpy. The two are seen spending time together in episodes such as I've Got You Under My Skin and A Hole Lotta Love. Sniffles also volunteers to help Lumpy in Blast from the Past. Also, in Pet Peeve, Sniffles greets Lumpy in the park and briefly shows concern when his "pet" injures Lumpy's leg.
However, their interactions do not always go smoothly, especially given Lumpy's low intelligence. Most notably in the episodes Wingin' It and Tongue in Cheek, Sniffles becomes irritated at Lumpy for his bumbling and stupidity. He (along with Handy and Flippy) also decide to throw Lumpy out of the rocket in Happy Trails Pt. 2: Jumping the Shark to remove excess weight. Also, in We're Scrooged!, he looks disappointed when Lumpy runs out of merchandise and does not seem concerned when Lumpy loses his eye. In Every Litter Bit Hurts, he teams up with Giggles to clean up the lake that Lumpy is polluting, choosing to indirectly go against him and also chaining himself to a tree to stop Lumpy from cutting it down. To be fair though, this is one of the episodes where Lumpy is in a directly villainous role.
The friendship appears to be slightly stronger on Lumpy's side, given that he usually takes care of Sniffles and the other Tree Friends. Lumpy also seems to have a degree of trust towards Sniffles, as can be seen in episodes like Blast from the Past. Even at times when they do not quite get along, such as Every Litter Bit Hurts and Wingin' It, Lumpy still becomes horrified and disgusted when he sees Sniffles being killed. In other episodes such as From A to Zoo and Take a Hike Lumpy also looks after Sniffles.
One exception would be Dream Job, where he seems to lack concern for Sniffles - implied as he watches television while the anteater sleeps and shows apathy to his deaths and injuries on the TV screen - and ends up driving him catatonic. As a giant in Dunce Upon a Time, he also captured Sniffles along with many other characters and kills him when he tries to escape.