The internet: a place where people take your beloved childhood memories and twist them with their dirty minds.
Let's assume, for the sake of argument, that SpongeBob does occasionally perform fellatio. Is he actually good at it?
In the first image Squidward appears to be having a pleasurable experience. The second image is unclear. Squidward seems to be struggling, but maybe he just ejaculated, and SpongeBob is continuing to suck on the penis, creating an intense and uncomfortable sensation.
Here SpongeBob is in a compromising position with Purple Doctorfish.
The doctor's expression indicates some initial trepidation followed by relieved pleasure.